6 PL/SQL Static SQL

CURRVAL and NEXTVAL

CURRVAL and NEXTVAL are sequence pseudocolumns. A sequence is a schema object that generates sequential numbers. When you create a sequence, you can specify its initial value and an increment. CURRVAL returns the current value in a specified sequence. Before you can reference CURRVAL in a session, you must use NEXTVAL to generate a number. A reference to NEXTVAL stores the current sequence number in CURRVAL. NEXTVAL increments the sequence and returns the next value. To get the current or next value in a sequence, use dot notation:

sequence_name.CURRVAL
sequence_name.NEXTVAL

The sequence_name can be either local or remote.

Each time you reference sequence_name.NEXTVAL, the sequence is incremented immediately and permanently, whether you commit or roll back the transaction.

Example 6-2 generates a sequence number and refers to that number in multiple statements. (The sequence must exist. Example 6-2 uses the sequence EMPLOYEES_SEQ, which is in the sample schema HR. To create a sequence, use the SQL statement CREATE SEQUENCE.)

DROP TABLE employees_temp;
CREATE TABLE employees_temp AS
  SELECT employee_id, first_name, last_name
  FROM employees;
 
DROP TABLE employees_temp2;
CREATE TABLE employees_temp2 AS
  SELECT employee_id, first_name, last_name
  FROM employees;
 
DECLARE
  seq_value NUMBER;
BEGIN
  -- Generate initial sequence number
 
  seq_value := employees_seq.NEXTVAL;
 
  -- Print initial sequence number:
 
  DBMS_OUTPUT.PUT_LINE (
    'Initial sequence value: ' || TO_CHAR(seq_value)
  );
 
  -- Use NEXTVAL to create unique number when inserting data:
 
     INSERT INTO employees_temp (employee_id, first_name, last_name) 
     VALUES (employees_seq.NEXTVAL, 'Lynette', 'Smith');
 
  -- Use CURRVAL to store same value somewhere else:
 
     INSERT INTO employees_temp2 VALUES (employees_seq.CURRVAL,
                                         'Morgan', 'Smith');
 
  /* Because NEXTVAL values might be referenced
     by different users and applications,
     and some NEXTVAL values might not be stored in database,
     there might be gaps in sequence. */
 
  -- Use CURRVAL to specify record to delete:
 
     seq_value := employees_seq.CURRVAL;
 
     DELETE FROM employees_temp2
     WHERE employee_id = seq_value;
 
  -- Udpate employee_id with NEXTVAL for specified record:
 
     UPDATE employees_temp
     SET employee_id = employees_seq.NEXTVAL
     WHERE first_name = 'Lynette'
     AND last_name = 'Smith';
 
  -- Display final value of CURRVAL:
 
     seq_value := employees_seq.CURRVAL;
 
     DBMS_OUTPUT.PUT_LINE (
       'Ending sequence value: ' || TO_CHAR(seq_value)
     );
END;
/

Usage Notes

• You can use sequence_name.CURRVAL and sequence_name.NEXTVAL wherever you can use a NUMBER expression.

• Using sequence_name.CURRVAL or sequence_name.NEXTVAL to provide an initial value for an ADT method parameter causes a compilation error.

• PL/SQL evaluates every occurrence of sequence_name.CURRVAL and sequence_name.NEXTVAL (unlike SQL, which evaluates a sequence expression once for every row in which it appears).

LEVEL

LEVEL is a hierarchical query pseudocolumn. If a table contains hierarchical data, you can select rows in a hierarchical order using the hierarchical query clause of the SELECT statement. For each row returned by a hierarchical query, the LEVEL pseudocolumn returns 1 for a root row, 2 for a child of a root, and so on.

ROWID

The ROWID pseudocolumn returns the rowid (binary address) of a row in a database table. You can use variables of type UROWID to store rowids in a readable format.

When you select or fetch a physical rowid into a UROWID variable, you can use the ROWIDTOCHAR function, which converts the binary value to a character string. You can compare the UROWID variable to the ROWID pseudocolumn in the WHERE clause of an UPDATE or DELETE statement to identify the latest row fetched from a cursor. For an example, see "Simulating CURRENT OF Clause with ROWID Pseudocolumn".

ROWNUM

For each row returned by a query, the ROWNUM pseudocolumn returns a number indicating the order in which Oracle Database selects the row from a table or set of joined rows. The first row selected has a ROWNUM of 1, the second has 2, and so on.

Implicit Cursors

PL/SQL opens an implicit cursor every time you run a SELECT or DML statement. You cannot control an implicit cursor, but you can get information from its attributes.

The syntax of an implicit cursor attribute value is SQLattribute (therefore, an implicit cursor is also called a SQL cursor). SQLattribute always refers to the most recently run SELECT or DML statement. If no such statement has run, the value of SQLattribute is NULL.

An implicit cursor closes after its associated statement runs; however, its attribute values remain available until another SELECT or DML statement runs.

The most recently run SELECT or DML statement might be in a different scope. To save an attribute value for later use, assign it to a local variable immediately. Otherwise, other operations, such as subprogram calls, might change the value of the attribute before you can test it.

The implicit cursor attributes are:

• SQL%ISOPEN Attribute: Is the Cursor Open?

• SQL%FOUND Attribute: Were Any Rows Affected?

• SQL%NOTFOUND Attribute: Were No Rows Affected?

• SQL%ROWCOUNT Attribute: How Many Rows Were Affected?

• SQL%BULK_ROWCOUNT (see "Counting Rows Affected by FORALL")

• SQL%BULK_EXCEPTIONS (see "Handling FORALL Exceptions")

DROP TABLE dept_temp;
CREATE TABLE dept_temp AS
  SELECT * FROM departments;
 
CREATE OR REPLACE PROCEDURE p (
  dept_no NUMBER
) AUTHID DEFINER AS
BEGIN
  DELETE FROM dept_temp
  WHERE department_id = dept_no;
 
  IF SQL%FOUND THEN
    DBMS_OUTPUT.PUT_LINE (
      'Delete succeeded for department number ' || dept_no
    );
  ELSE
    DBMS_OUTPUT.PUT_LINE ('No department number ' || dept_no);
  END IF;
END;
/
BEGIN
  p(270);
  p(400);
END;
/

DROP TABLE employees_temp;
CREATE TABLE employees_temp AS
  SELECT * FROM employees;

DECLARE
  mgr_no NUMBER(6) := 122;
BEGIN
  DELETE FROM employees_temp WHERE manager_id = mgr_no;
  DBMS_OUTPUT.PUT_LINE
    ('Number of employees deleted: ' || TO_CHAR(SQL%ROWCOUNT));
END;
/

Number of employees deleted: 8

Explicit Cursors

You must declare an explicit cursor, giving it a name and associating it with a query—usually one that returns multiple rows. After declaring an explicit cursor, you must open it (with the OPEN statement), fetch rows one at a time from the result set (with the FETCH statement), and close the cursor (with the CLOSE statement). After closing the cursor, you can neither fetch records from the result set nor see the cursor attribute values.

Topics:

• Declaring Explicit Cursors

• Opening Explicit Cursors

• Fetching Data with Explicit Cursors

• Fetching Bulk Data with Explicit Cursors

• Closing Explicit Cursors

• Explicit Cursor Attributes

• When Explicit Cursors Need Column Aliases

• Explicit Cursors that Accept Parameters

DECLARE
  my_emp_id  NUMBER(6);     -- variable for employee_id
  my_job_id  VARCHAR2(10);  -- variable for job_id
  my_sal     NUMBER(8,2);   -- variable for salary

  CURSOR c1 IS
    SELECT employee_id, job_id, salary FROM employees
    WHERE salary > 2000; 

  my_dept   departments%ROWTYPE;  -- variable for departments row

  CURSOR c2 RETURN departments%ROWTYPE IS
    SELECT * FROM departments
    WHERE department_id = 110;
BEGIN
  NULL;
END;
/

DECLARE
   CURSOR c1 (low  NUMBER := 0, high NUMBER := 99) IS
              SELECT * FROM departments WHERE department_id > low
              AND department_id < high;

DECLARE
  CURSOR c1 IS
    SELECT employee_id, last_name, job_id, salary
    FROM employees
    WHERE salary > 2000;
BEGIN
  OPEN c1;
END;
/

DECLARE
  v_jobid     employees.job_id%TYPE;     -- variable for job_id
  v_lastname  employees.last_name%TYPE;  -- variable for last_name

  CURSOR c1 IS
    SELECT last_name, job_id FROM employees
    WHERE REGEXP_LIKE (job_id, 'S[HT]_CLERK');

  v_employees employees%ROWTYPE;         -- record variable for row

  CURSOR c2 IS
    SELECT * FROM employees
    WHERE REGEXP_LIKE (job_id, '[ACADFIMKSA]_M[ANGR]');

BEGIN
  OPEN c1;
  LOOP  -- Fetches 2 columns into variables
    FETCH c1 INTO v_lastname, v_jobid;
    EXIT WHEN c1%NOTFOUND;
    DBMS_OUTPUT.PUT_LINE( RPAD(v_lastname, 25, ' ') || v_jobid );
  END LOOP;
  CLOSE c1;
  DBMS_OUTPUT.PUT_LINE( '-------------------------------------' );

  OPEN c2;
  LOOP  -- Fetches entire row into the v_employees record
    FETCH c2 INTO v_employees;
    EXIT WHEN c2%NOTFOUND;
    DBMS_OUTPUT.PUT_LINE( RPAD(v_employees.last_name, 25, ' ') ||
                               v_employees.job_id );
  END LOOP;
  CLOSE c2;
END;
/

Nayer                    ST_CLERK
Mikkilineni              ST_CLERK
Landry                   ST_CLERK
...
Higgins                  AC_MGR

DECLARE
  my_sal employees.salary%TYPE;
  my_job employees.job_id%TYPE;
  factor INTEGER := 2;
  CURSOR c1 IS
    SELECT factor*salary FROM employees
    WHERE job_id = my_job;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO my_sal;
    EXIT WHEN c1%NOTFOUND;
    factor := factor + 1;  -- does not affect FETCH
  END LOOP;
  CLOSE c1;
END;
/

DECLARE
  CURSOR c1 IS
    SELECT last_name FROM employees
    ORDER BY last_name;

  name1 employees.last_name%TYPE;
  name2 employees.last_name%TYPE;
  name3 employees.last_name%TYPE;
BEGIN
  OPEN c1;
  FETCH c1 INTO name1;  -- fetches first row
  FETCH c1 INTO name2;  -- fetches second row
  FETCH c1 INTO name3;  -- fetches third row
  CLOSE c1;
END;
/

DECLARE
  TYPE IdsTab  IS TABLE OF employees.employee_id%TYPE;
  TYPE NameTab IS TABLE OF employees.last_name%TYPE;

  ids    IdsTab;
  names  NameTab;

  CURSOR c1 IS
    SELECT employee_id, last_name
    FROM employees
    WHERE job_id = 'ST_CLERK';

BEGIN
  OPEN c1;
  FETCH c1 BULK COLLECT INTO ids, names;
  CLOSE c1;
  -- Process collection elements
  FOR i IN ids.FIRST .. ids.LAST
    LOOP
      IF ids(i) > 140 THEN
        DBMS_OUTPUT.PUT_LINE( ids(i) );
      END IF;
    END LOOP;
  FOR i IN names.FIRST .. names.LAST
    LOOP
      IF names(i) LIKE '%Ma%' THEN
        DBMS_OUTPUT.PUT_LINE( names(i) );
      END IF;
    END LOOP;
END;
/

141
142
143
144
Markle
Marlow
Mallin
Matos

DECLARE
  CURSOR c1 IS
    SELECT last_name, salary FROM employees
    WHERE ROWNUM < 11;

  the_name employees.last_name%TYPE;
  the_salary employees.salary%TYPE;
BEGIN
  IF c1%ISOPEN = FALSE THEN
    OPEN c1;
  END IF;
  FETCH c1 INTO the_name, the_salary;
  CLOSE c1;
END;
/

DECLARE
  CURSOR c1 IS
    SELECT last_name, salary FROM employees
    WHERE ROWNUM < 11;

  my_ename   employees.last_name%TYPE;
  my_salary  employees.salary%TYPE;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO my_ename, my_salary;
    IF c1%FOUND THEN  -- fetch succeeded
      DBMS_OUTPUT.PUT_LINE('Name = ' || my_ename || ', salary = ' || my_salary);
    ELSE  -- fetch failed
      EXIT;
    END IF;
  END LOOP;
END;
/

Name = King, salary = 24100
Name = Kochhar, salary = 17000
Name = De Haan, salary = 17000
Name = Hunold, salary = 9100
Name = Ernst, salary = 6000
Name = Austin, salary = 4800
Name = Pataballa, salary = 4800
Name = Lorentz, salary = 4300
Name = Greenberg, salary = 12008
Name = Faviet, salary = 9000

DECLARE
  CURSOR c1 IS
    SELECT last_name, salary FROM employees
    WHERE ROWNUM < 11;

   my_ename   employees.last_name%TYPE;
   my_salary  employees.salary%TYPE;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO my_ename, my_salary;
    IF c1%NOTFOUND THEN -- fetch failed
    -- Another form of this test is
    -- "EXIT WHEN c1%NOTFOUND OR c1%NOTFOUND IS NULL;"
      EXIT;
    ELSE  -- fetch succeeded
      DBMS_OUTPUT.PUT_LINE
        ('Name = ' || my_ename || ', salary = ' || my_salary);
    END IF;
  END LOOP;
END;
/

Name = King, salary = 24100
Name = Kochhar, salary = 17000
Name = De Haan, salary = 17000
Name = Hunold, salary = 9100
Name = Ernst, salary = 6000
Name = Austin, salary = 4800
Name = Pataballa, salary = 4800
Name = Lorentz, salary = 4300
Name = Greenberg, salary = 12008
Name = Faviet, salary = 9000

EXIT WHEN c1%NOTFOUND OR (c1%NOTFOUND IS NULL);

DECLARE
  CURSOR c1 IS
    SELECT last_name FROM employees
    WHERE ROWNUM < 11;

  name  employees.last_name%TYPE;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO name;
    EXIT WHEN c1%NOTFOUND OR c1%NOTFOUND IS NULL;
    DBMS_OUTPUT.PUT_LINE(c1%ROWCOUNT || '. ' || name);
    IF c1%ROWCOUNT = 5 THEN
       DBMS_OUTPUT.PUT_LINE('--- Fetched 5th record ---');
    END IF;
  END LOOP;
  CLOSE c1;
END;
/

1. Abel
2. Ande
3. Atkinson
4. Austin
5. Baer
--- Fetched 5th record ---
6. Baida
7. Banda
8. Bates
9. Bell
10. Bernstein

DECLARE
  CURSOR c1 IS
    SELECT employee_id,
           (salary * .05) raise
    FROM employees
    WHERE job_id LIKE '%_MAN';
  emp_rec c1%ROWTYPE;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO emp_rec;
    EXIT WHEN c1%NOTFOUND;
    DBMS_OUTPUT.PUT_LINE (
      'Raise for employee #' || emp_rec.employee_id ||
      ' is $' || emp_rec.raise
    ); 
  END LOOP;
  CLOSE c1;
END;
/

DECLARE
  CURSOR c1 (job VARCHAR2, max_wage NUMBER) IS
    SELECT * FROM employees
    WHERE job_id = job
    AND salary > max_wage;
BEGIN
  FOR person IN c1('CLERK', 3000)
  LOOP
     -- process data record
    DBMS_OUTPUT.PUT_LINE (
      'Name = ' || person.last_name || ', salary = ' ||
      person.salary || ', Job Id = ' || person.job_id
    );
  END LOOP;
END;
/

DECLARE
  emp_job     employees.job_id%TYPE := 'ST_CLERK';
  emp_salary  employees.salary%TYPE := 3000;
  my_record   employees%ROWTYPE;

  CURSOR c1 (job VARCHAR2, max_wage NUMBER) IS
    SELECT * FROM employees
    WHERE job_id = job
    AND salary > max_wage;
BEGIN
/* Open the cursor with one of these statements:
   OPEN c1('ST_CLERK', 3000);
   OPEN c1('ST_CLERK', emp_salary);
   OPEN c1(emp_job, 3000);
   OPEN c1(emp_job, emp_salary); */

  OPEN c1(emp_job, emp_salary);

  LOOP
     FETCH c1 INTO my_record;
     EXIT WHEN c1%NOTFOUND;
     DBMS_OUTPUT.PUT_LINE
       ('Name = ' || my_record.last_name || ', salary = ' ||
        my_record.salary || ', Job Id = ' || my_record.job_id );
  END LOOP;
END;
/

Name = Nayer, salary = 3200, Job Id = ST_CLERK
Name = Bissot, salary = 3300, Job Id = ST_CLERK
Name = Mallin, salary = 3300, Job Id = ST_CLERK
Name = Ladwig, salary = 3600, Job Id = ST_CLERK
Name = Stiles, salary = 3200, Job Id = ST_CLERK
Name = Rajs, salary = 3500, Job Id = ST_CLERK
Name = Davies, salary = 3100, Job Id = ST_CLERK

Selecting At Most One Row (SELECT INTO Statement)

If you expect a query to only return one row, you can write a regular SQL SELECT statement with an additional INTO clause specifying the PL/SQL variable to hold the result.

If the query might return multiple rows, but you do not care about values after the first, you can restrict any result set to a single row by comparing the ROWNUM value. If the query might return no rows at all, use an exception handler to specify any actions to take when no data is found.

If you just want to check whether a condition exists in your data, you might be able to code the query with the COUNT(*) operator, which always returns a number and never raises the NO_DATA_FOUND exception.

For more information about the SELECT INTO statement, see "SELECT INTO Statement".

Selecting Multiple Rows (BULK COLLECT Clause)

If you must bring a large quantity of data into local PL/SQL variables, rather than looping through a result set one row at a time, you can use the BULK COLLECT clause. When you query only certain columns, you can store all the results for each column in a separate collection variable. When you query all the columns of a table, you can store the entire result set in a collection of records, which makes it convenient to loop through the results and refer to different columns. See Example 6-10, "Fetching Bulk Data with a Cursor".

This technique can be very fast, but also very memory-intensive. If you use it often, you might be able to improve your code by doing more of the work in SQL:

• If you must loop once through the result set, use a FOR LOOP as described in the following sections. This technique avoids the memory overhead of storing a copy of the result set.

• If you are looping through the result set to scan for certain values or filter the results into a smaller set, do this scanning or filtering in the original query instead. You can add more WHERE clauses in simple cases, or use set operators such as INTERSECT and MINUS if you are comparing two or more sets of results.

• If you are looping through the result set and running another query or a DML statement for each result row, you can probably find a more efficient technique. For queries, look at including subqueries or EXISTS or NOT EXISTS clauses in the original query. For DML statements, look at the FORALL statement, which is much faster than coding these statements inside a regular loop.

Looping Through Multiple Rows (Cursor FOR LOOP Statement)

Perhaps the most common case of a query is one where you issue the SELECT statement, then immediately loop through the rows of the result set. PL/SQL lets you use a cursor FOR LOOP for this kind of query.

The iterator variable for the cursor FOR LOOP does need not be declared in advance. It is a %ROWTYPE record whose field names match the column names from the query, and that exists only during the loop. When you use expressions rather than explicit column names, use column aliases so that you can refer to the corresponding values inside the loop.

Topics:

• Implicit Cursor FOR LOOP

• Explicit Cursor FOR LOOP

• Column Aliases for Expression Values in Cursor FOR LOOP

BEGIN
  FOR item IN (
    SELECT last_name, job_id
    FROM employees
    WHERE job_id LIKE '%CLERK%'
    AND manager_id > 120
  )
  LOOP
    DBMS_OUTPUT.PUT_LINE
      ('Name = ' || item.last_name || ', Job = ' || item.job_id);
  END LOOP;
END;
/

Name = Bissot, Job = ST_CLERK
Name = Atkinson, Job = ST_CLERK
Name = Marlow, Job = ST_CLERK
...
Name = Grant, Job = SH_CLERK

DECLARE
  CURSOR c1 IS
    SELECT last_name, job_id FROM employees
    WHERE job_id LIKE '%CLERK%' AND manager_id > 120;
BEGIN
  FOR item IN c1
  LOOP
    DBMS_OUTPUT.PUT_LINE
      ('Name = ' || item.last_name || ', Job = ' || item.job_id);
  END LOOP;
END;
/

Name = Bissot, Job = ST_CLERK
Name = Atkinson, Job = ST_CLERK
Name = Marlow, Job = ST_CLERK
...
Name = Grant, Job = SH_CLERK

BEGIN
  FOR item IN (
    SELECT first_name || ' ' || last_name AS full_name,
           salary * 10                    AS dream_salary 
    FROM employees
    WHERE ROWNUM <= 5
  ) LOOP
    DBMS_OUTPUT.PUT_LINE
      (item.full_name || ' dreams of making ' || item.dream_salary);
  END LOOP;
END;
/

Steven King dreams of making 241000
Neena Kochhar dreams of making 170000
Lex De Haan dreams of making 170000
Alexander Hunold dreams of making 91000
Bruce Ernst dreams of making 60000

Processing Complicated Queries with Explicit Cursors

For full control over query processing, you can use explicit cursors in combination with the OPEN, FETCH, and CLOSE statements.

You might want to specify a query in one place but retrieve the rows somewhere else, even in another subprogram. Or you might want to choose very different query parameters, such as ORDER BY or GROUP BY clauses, depending on the situation. Or you might want to process some rows differently than others, and so need more than a simple loop.

Because explicit cursors are so flexible, you can choose from different notations depending on your needs. The following sections describe all the query-processing features that explicit cursors provide.

For more information about explicit cursors, see "Explicit Cursors".

Purpose of Cursor Variables

You use cursor variables to pass query result sets between PL/SQL stored subprograms and their clients. This is possible because PL/SQL and its clients share a pointer to the work area where the result set is stored.

Cursor variables are available to every PL/SQL client. For example, you can declare a cursor variable in a PL/SQL host environment such as an OCI or Pro*C program, then pass it as an input host variable (bind variable) to PL/SQL. Application development tools such as Oracle Forms, which have a PL/SQL engine, can use cursor variables entirely on the client side. Or, you can pass cursor variables back and forth between a client and the database server through remote subprogram calls.

A work area remains accessible while any cursor variable points to it, even if you pass the value of a cursor variable from one scope to another. For example, if you pass a host cursor variable to a PL/SQL block embedded in a Pro*C program, the work area to which the cursor variable points remains accessible after the block completes.

If you have a PL/SQL engine on the client side, calls from client to server impose no restrictions. For example, you can declare a cursor variable on the client side, open and fetch from it on the server side, and continue to fetch from it back on the client side. You can also reduce network traffic by having a PL/SQL block open or close several host cursor variables in a single round trip.

Cursor variables are also useful for passing query results between subprograms. Opening the cursor variable in one subprogram and processing it in a different subprogram helps to centralize data retrieval. This technique is also useful for multilanguage applications, where a PL/SQL subprogram might return a result set to a subprogram written in a different language, such as Java or Visual Basic.

Cursor Variable Creation

To create a cursor variable, you define a REF CURSOR type, and then declare a cursor variable of that type.

You can define REF CURSOR types in any PL/SQL block, subprogram, or package. In this example, you declare a REF CURSOR type that represents a result set from the DEPARTMENTS table:

DECLARE
  TYPE DeptCurTyp IS REF CURSOR RETURN departments%ROWTYPE;
BEGIN
  NULL;
END;
/

REF CURSOR types can be strong (with a return type) or weak (with no return type). Strong REF CURSOR types are less error-prone because the PL/SQL compiler lets you associate a strongly typed cursor variable only with queries that return the right set of columns. Weak REF CURSOR types are more flexible because the compiler lets you associate a weakly typed cursor variable with any query. Because there is no type checking with a weak REF CURSOR, all such types are interchangeable. Instead of creating a type, you can use the predefined type SYS_REFCURSOR.

If both cursor variables involved in an assignment are strongly typed, they must have the same data type (not just the same return type). If one or both cursor variables are weakly typed, they can have different data types.

After defining a REF CURSOR type, you can declare cursor variables of that type in any PL/SQL block or subprogram.

DECLARE
  TYPE empcurtyp IS REF CURSOR RETURN employees%ROWTYPE;  -- strong
  TYPE genericcurtyp IS REF CURSOR;  -- weak

  cursor1  empcurtyp;
  cursor2  genericcurtyp;
  my_cursor SYS_REFCURSOR;  -- no new type needed
  TYPE deptcurtyp IS REF CURSOR RETURN departments%ROWTYPE;
  dept_cv deptcurtyp;  -- declare cursor variable
BEGIN
  NULL;
END;
/

To avoid declaring the same REF CURSOR type in each subprogram that uses it, you can put the REF CURSOR declaration in a package specification. You can declare cursor variables of that type in the corresponding package body, or in your own subprogram.

In the RETURN clause of a REF CURSOR type definition, you can use %ROWTYPE to refer to a strongly typed cursor variable, as shown in Example 6-25.

DECLARE
  TYPE TmpCurTyp IS REF CURSOR RETURN employees%ROWTYPE;
  tmp_cv TmpCurTyp;

  TYPE EmpCurTyp IS REF CURSOR RETURN tmp_cv%ROWTYPE;
  emp_cv EmpCurTyp;
BEGIN
  NULL;
END;
/

You can also use %ROWTYPE to provide the data type of a record variable, as shown in Example 6-26.

DECLARE
  dept_rec departments%ROWTYPE;  -- record variable
  TYPE DeptCurTyp IS REF CURSOR RETURN dept_rec%TYPE;
  dept_cv DeptCurTyp;  -- cursor variable
BEGIN
  NULL;
END;
/

Example 6-27 specifies a user-defined RECORD type in the RETURN clause.

DECLARE
  TYPE EmpRecTyp IS RECORD (
    employee_id NUMBER,
    last_name VARCHAR2(25),
    salary   NUMBER(8,2));

  TYPE EmpCurTyp IS REF CURSOR RETURN EmpRecTyp;
  emp_cv EmpCurTyp;
BEGIN
  NULL;
END;
/

Cursor Variables as Subprogram Parameters

You can declare cursor variables as the formal parameters of subprograms.

When you declare a cursor variable as the formal parameter of a subprogram that fetches from the cursor variable, you must specify the IN or IN OUT mode. However, if the subprogram also opens the cursor variable, you must specify the IN OUT mode.

Example 6-28 defines a REF CURSOR type and then declares a cursor variable of that type as a formal parameter.

DECLARE
  TYPE empcurtyp IS REF CURSOR RETURN employees%ROWTYPE;
  emp  empcurtyp;

  /* After building result set,
     process all rows inside a single procedure,
     rather than invoking a procedure for each row. */

  PROCEDURE process_emp_cv (emp_cv IN empcurtyp) IS
    person employees%ROWTYPE;
  BEGIN
    DBMS_OUTPUT.PUT_LINE ('-----');
    DBMS_OUTPUT.PUT_LINE ('Here are the names from the result set:');

    LOOP
      FETCH emp_cv INTO person;
      EXIT WHEN emp_cv%NOTFOUND;
      DBMS_OUTPUT.PUT_LINE ('Name = ' || person.first_name ||
                            ' ' || person.last_name);
    END LOOP;
  END;
BEGIN
  -- Find 10 arbitrary employees.
  OPEN emp FOR SELECT * FROM employees WHERE ROWNUM < 11;
  process_emp_cv(emp);
  CLOSE emp;

  -- Find employees matching a condition.
  OPEN emp FOR SELECT * FROM employees WHERE last_name LIKE 'R%';
  process_emp_cv(emp);
  CLOSE emp;
END;
/

-----
Here are the names from the result set:
Name = Steven King
Name = Neena Kochhar
Name = Lex De Haan
Name = Alexander Hunold
Name = Bruce Ernst
Name = David Austin
Name = Valli Pataballa
Name = Diana Lorentz
Name = Nancy Greenberg
Name = Daniel Faviet
-----
Here are the names from the result set:
Name = Trenna Rajs
Name = Den Raphaely
Name = Michael Rogers
Name = John Russell

Like all pointers, cursor variables increase the possibility of parameter aliasing. See "Overloaded Subprograms".

Be careful when passing cursor variables as parameters. At run time, PL/SQL raises ROWTYPE_MISMATCH if the return types of the actual and formal parameters are incompatible.

Cursor Variable Control (OPEN FOR, FETCH, and CLOSE Statements)

You use three statements to control a cursor variable: OPEN FOR, FETCH, and CLOSE. First, you OPEN a cursor variable FOR a multiple-row query. Then, you FETCH rows from the result set. When all the rows are processed, you CLOSE the cursor variable.

Topics:

• Opening Cursor Variables

• Fetching from Cursor Variables

• Closing Cursor Variables

• INVALID_CURSOR Exception

DECLARE
  TYPE empcurtyp IS REF CURSOR RETURN employees%ROWTYPE;
  emp_cv empcurtyp;
BEGIN
  IF NOT emp_cv%ISOPEN THEN
    OPEN emp_cv FOR SELECT * FROM employees;
  END IF;
  CLOSE emp_cv;
END;
/

CREATE OR REPLACE PACKAGE emp_data AS
  TYPE empcurtyp IS REF CURSOR RETURN employees%ROWTYPE;
  PROCEDURE open_emp_cv (emp_cv IN OUT empcurtyp);
END emp_data;
/
CREATE OR REPLACE PACKAGE BODY emp_data AS
  PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp) IS
  BEGIN
    OPEN emp_cv FOR SELECT * FROM employees;
  END open_emp_cv;
END emp_data;
/

CREATE OR REPLACE PACKAGE emp_data AS
  TYPE empcurtyp IS REF CURSOR RETURN employees%ROWTYPE;
  PROCEDURE open_emp_cv (emp_cv IN OUT empcurtyp, choice INT);
END emp_data;
/
CREATE OR REPLACE PACKAGE BODY emp_data AS
  PROCEDURE open_emp_cv (emp_cv IN OUT empcurtyp, choice INT) IS
  BEGIN
    IF choice = 1 THEN
      OPEN emp_cv FOR SELECT *
      FROM employees
      WHERE commission_pct IS NOT NULL;
    ELSIF choice = 2 THEN
      OPEN emp_cv FOR SELECT *
      FROM employees
      WHERE salary > 2500;
    ELSIF choice = 3 THEN
      OPEN emp_cv FOR SELECT *
      FROM employees
      WHERE department_id = 100;
    END IF;
  END;
END emp_data;
/

CREATE OR REPLACE PACKAGE admin_data AS
  TYPE gencurtyp IS REF CURSOR;
  PROCEDURE open_cv (generic_cv IN OUT gencurtyp, choice INT);
END admin_data;
/
CREATE OR REPLACE PACKAGE BODY admin_data AS
  PROCEDURE open_cv (generic_cv IN OUT gencurtyp, choice INT) IS
  BEGIN
    IF choice = 1 THEN
      OPEN generic_cv FOR SELECT * FROM employees;
    ELSIF choice = 2 THEN
      OPEN generic_cv FOR SELECT * FROM departments;
    ELSIF choice = 3 THEN
      OPEN generic_cv FOR SELECT * FROM jobs;
    END IF;
  END;
END admin_data;
/

DECLARE
  TYPE empcurtyp IS REF CURSOR RETURN employees%ROWTYPE;
  emp_cv empcurtyp;
  emp_rec employees%ROWTYPE;
BEGIN
  OPEN emp_cv FOR SELECT * FROM employees WHERE employee_id < 120;
  LOOP
    FETCH emp_cv INTO emp_rec; -- fetch from cursor variable
    EXIT WHEN emp_cv%NOTFOUND; -- exit when last row is fetched
    -- process data record
    DBMS_OUTPUT.PUT_LINE
      ('Name = ' || emp_rec.first_name || ' ' || emp_rec.last_name);
  END LOOP;
  CLOSE emp_cv;
END;
/

Name = Steven King
Name = Neena Kochhar
Name = Lex De Haan
...
Name = Karen Colmenares

DECLARE
  TYPE empcurtyp IS REF CURSOR;
  TYPE namelist IS TABLE OF employees.last_name%TYPE;
  TYPE sallist IS TABLE OF employees.salary%TYPE;
  emp_cv  empcurtyp;
  names   namelist;
  sals    sallist;
BEGIN
  OPEN emp_cv FOR
    SELECT last_name, salary FROM employees
    WHERE job_id = 'SA_REP';

  FETCH emp_cv BULK COLLECT INTO names, sals;
  CLOSE emp_cv;
  -- loop through the names and sals collections
  FOR i IN names.FIRST .. names.LAST
  LOOP
    DBMS_OUTPUT.PUT_LINE
      ('Name = ' || names(i) || ', salary = ' || sals(i));
  END LOOP;
END;
/

Name = Tucker, salary = 10000
Name = Bernstein, salary = 9500
Name = Hall, salary = 9000
...
Name = Johnson, salary = 6200

Cursor Variables as Host Variables

You can declare a cursor variable in a PL/SQL host environment such as an OCI or Pro*C program. To use the cursor variable, you must pass it as a host variable to PL/SQL.

Example 6-35, a Pro*C program passes a host cursor variable and selector to a PL/SQL block, which opens the cursor variable for the chosen query.

EXEC SQL BEGIN DECLARE SECTION;
  SQL_CURSOR  generic_cv;  -- host cursor variable
  int         choice;
EXEC SQL END DECLARE SECTION;
EXEC SQL ALLOCATE :generic_cv;  -- Initialize host cursor variable.
-- Pass host cursor variable and selector to PL/SQL block.
/
EXEC SQL EXECUTE
BEGIN
  IF :choice = 1 THEN
    OPEN :generic_cv FOR SELECT * FROM employees;
  ELSIF :choice = 2 THEN
    OPEN :generic_cv FOR SELECT * FROM departments;
  ELSIF :choice = 3 THEN
    OPEN :generic_cv FOR SELECT * FROM jobs;
  END IF;
END;
END-EXEC;

Host cursor variables are compatible with any query return type. They act like weakly typed PL/SQL cursor variables.

When passing host cursor variables to PL/SQL, you can reduce network traffic by grouping OPEN FOR statements, as in Example 6-36, where the PL/SQL block opens multiple cursor variables in a single round trip.

/* anonymous PL/SQL block in host environment */
BEGIN
  OPEN :emp_cv FOR SELECT * FROM employees;
  OPEN :dept_cv FOR SELECT * FROM departments;
  OPEN :loc_cv FOR SELECT * FROM locations;
END;
/

This technique can be useful when you want to populate a multiblock form, as in in Oracle Forms. When you pass host cursor variables to a PL/SQL block for opening, the query work areas to which they point remain accessible after the block completes, so your OCI or Pro*C program can use these work areas for ordinary cursor operations. For example, you open several such work areas in a single round trip:

BEGIN
  OPEN :c1 FOR SELECT 1 FROM DUAL;
  OPEN :c2 FOR SELECT 1 FROM DUAL;
  OPEN :c3 FOR SELECT 1 FROM DUAL;
END;
/

The cursors assigned to c1, c2, and c3 act normally, and you can use them for any purpose. When finished, release the cursors as follows:

BEGIN
  CLOSE :c1; CLOSE :c2; CLOSE :c3;
END;
/

Cursor Variable Restrictions

Cursor variables are subject to these restrictions:

• You cannot declare cursor variables in a package specification, as Example 6-37 shows.

• If you bind a host cursor variable into PL/SQL from an Oracle Call Interface (OCI) client, you cannot fetch from it on the server side unless you also open it there on the same server call.

• You cannot use comparison operators to test cursor variables for equality, inequality, or nullity.

• Database columns cannot store the values of cursor variables. There is no equivalent type to use in a CREATE TABLE statement.

• You cannot store cursor variables in a collection.

• Cursors and cursor variables are not interchangeable (you cannot use one where the other is expected). For example, you cannot reference a cursor variable in a cursor FOR LOOP.

• A cursor variable is permitted in a server-to-server RPC only if the remote database is not an Oracle Database accessed through a Procedural Gateway.

CREATE OR REPLACE PACKAGE emp_data AS
  TYPE EmpCurTyp IS REF CURSOR RETURN employees%ROWTYPE;
  -- emp_cv EmpCurTyp; -- not allowed
  PROCEDURE open_emp_cv;
END emp_data;
/
CREATE OR REPLACE PACKAGE BODY emp_data AS
  -- emp_cv EmpCurTyp; -- not allowed
  PROCEDURE open_emp_cv IS
    emp_cv EmpCurTyp; -- this is legal
  BEGIN
    OPEN emp_cv FOR SELECT * FROM employees;
  END open_emp_cv;
END emp_data;
/

COMMIT Statement

The COMMIT statement ends the current transaction, making any changes made during that transaction permanent and visible to other users. Transactions are not tied to PL/SQL BEGIN END blocks. A block can contain multiple transactions, and a transaction can span multiple blocks.

Asynchronous commit provides more control for the user with the WRITE clause. This option specifies the priority with which the redo information generated by the commit operation is written to the redo log.

In Example 6-39, a transaction transfers money from one bank account to another. It is important that the money both leaves one account and enters the other, hence the COMMIT WRITE IMMEDIATE NOWAIT statement.

DROP TABLE accounts;
CREATE TABLE accounts (
  account_id  NUMBER(6),
  balance     NUMBER (10,2)
);
 
INSERT INTO accounts (account_id, balance)
VALUES (7715, 6350.00);
 
INSERT INTO accounts (account_id, balance)
VALUES (7720, 5100.50);
 
CREATE OR REPLACE PROCEDURE transfer (
  from_acct  NUMBER,
  to_acct    NUMBER,
  amount     NUMBER
) AUTHID DEFINER AS
BEGIN
  UPDATE accounts
  SET balance = balance - amount
  WHERE account_id = from_acct;
 
  UPDATE accounts
  SET balance = balance + amount
  WHERE account_id = to_acct;
 
  COMMIT WRITE IMMEDIATE NOWAIT;
END;
/

SELECT * FROM accounts;

ACCOUNT_ID    BALANCE
---------- ----------
      7715       6350
      7720     5100.5
 
BEGIN
  transfer(7715, 7720, 250);
END;
/
 

SELECT * FROM accounts;

ACCOUNT_ID    BALANCE
---------- ----------
      7715       6100
      7720     5350.5

ROLLBACK Statement

The ROLLBACK statement ends the current transaction and undoes any changes made during that transaction. If you make a mistake, such as deleting the wrong row from a table, a rollback restores the original data. If you cannot finish a transaction because an exception is raised or a SQL statement fails, a rollback lets you take corrective action and perhaps start over.

Example 6-40 inserts information about an employee into three different tables. If an INSERT statement tries to store a duplicate employee number, the predefined exception DUP_VAL_ON_INDEX is raised. To ensure that changes to all three tables are undone, the exception handler runs a ROLLBACK.

DROP TABLE emp_name;
CREATE TABLE emp_name AS 
  SELECT employee_id, last_name
  FROM employees;
 
CREATE UNIQUE INDEX empname_ix
ON emp_name (employee_id);
 
 
DROP TABLE emp_sal;
CREATE TABLE emp_sal AS
  SELECT employee_id, salary
  FROM employees;
 
CREATE UNIQUE INDEX empsal_ix
ON emp_sal (employee_id);
 
 
DROP TABLE emp_job;
CREATE TABLE emp_job AS
  SELECT employee_id, job_id
  FROM employees;
 
CREATE UNIQUE INDEX empjobid_ix
ON emp_job (employee_id);
 
 
DECLARE
  emp_id        NUMBER(6);
  emp_lastname  VARCHAR2(25);
  emp_salary    NUMBER(8,2);
  emp_jobid     VARCHAR2(10);
BEGIN
  SELECT employee_id, last_name, salary, job_id
  INTO emp_id, emp_lastname, emp_salary, emp_jobid
  FROM employees
  WHERE employee_id = 120;
 
  INSERT INTO emp_name (employee_id, last_name)
  VALUES (emp_id, emp_lastname);
 
  INSERT INTO emp_sal (employee_id, salary) 
  VALUES (emp_id, emp_salary);
 
  INSERT INTO emp_job (employee_id, job_id)
  VALUES (emp_id, emp_jobid);
 
EXCEPTION
  WHEN DUP_VAL_ON_INDEX THEN
    ROLLBACK;
    DBMS_OUTPUT.PUT_LINE('Inserts were rolled back');
END;
/

SAVEPOINT Statement

The SAVEPOINT statement names and marks the current point in the processing of a transaction. Savepoints let you roll back part of a transaction instead of the whole transaction. The number of active savepoints for each session is unlimited.

Example 6-41 marks a savepoint before doing an insert. If the INSERT statement tries to store a duplicate value in the employee_id column, the predefined exception DUP_VAL_ON_INDEX is raised and the transaction rolls back to the savepoint, undoing only the INSERT statement.

DROP TABLE emp_name;
CREATE TABLE emp_name AS
  SELECT employee_id, last_name, salary
  FROM employees;
 
CREATE UNIQUE INDEX empname_ix
ON emp_name (employee_id);
 
DECLARE
  emp_id        employees.employee_id%TYPE;
  emp_lastname  employees.last_name%TYPE;
  emp_salary    employees.salary%TYPE;
 
BEGIN
  SELECT employee_id, last_name, salary
  INTO emp_id, emp_lastname, emp_salary 
  FROM employees
  WHERE employee_id = 120;
 
  UPDATE emp_name
  SET salary = salary * 1.1
  WHERE employee_id = emp_id;
 
  DELETE FROM emp_name
  WHERE employee_id = 130;
 
  SAVEPOINT do_insert;
 
  INSERT INTO emp_name (employee_id, last_name, salary)
  VALUES (emp_id, emp_lastname, emp_salary);
 
EXCEPTION
  WHEN DUP_VAL_ON_INDEX THEN
    ROLLBACK TO do_insert;
  DBMS_OUTPUT.PUT_LINE('Insert was rolled back');
END;
/

When you roll back to a savepoint, any savepoints marked after that savepoint are erased. The savepoint to which you roll back is not erased. A simple rollback or commit erases all savepoints.

If you mark a savepoint in a recursive subprogram, new instances of the SAVEPOINT statement run at each level in the recursive descent, but you can only roll back to the most recently marked savepoint.

Savepoint names are undeclared identifiers. Reusing a savepoint name in a transaction moves the savepoint from its old position to the current point in the transaction, which means that a rollback to the savepoint affects only the current part of the transaction.

DROP TABLE emp_name;
CREATE TABLE emp_name AS
  SELECT employee_id, last_name, salary
  FROM employees;
 
CREATE UNIQUE INDEX empname_ix
ON emp_name (employee_id);
 
DECLARE
  emp_id        employees.employee_id%TYPE;
  emp_lastname  employees.last_name%TYPE;
  emp_salary    employees.salary%TYPE;
 
BEGIN
  SELECT employee_id, last_name, salary
  INTO emp_id, emp_lastname, emp_salary
  FROM employees
  WHERE employee_id = 120;
 
  SAVEPOINT my_savepoint;
 
  UPDATE emp_name
  SET salary = salary * 1.1
  WHERE employee_id = emp_id;
 
  DELETE FROM emp_name
  WHERE employee_id = 130;
 
  SAVEPOINT my_savepoint;
 
  INSERT INTO emp_name (employee_id, last_name, salary)
  VALUES (emp_id, emp_lastname, emp_salary);
 
EXCEPTION
  WHEN DUP_VAL_ON_INDEX THEN
    ROLLBACK TO my_savepoint;
    DBMS_OUTPUT.PUT_LINE('Transaction rolled back.');
END;
/

Implicit Rollbacks

Before running an INSERT, UPDATE, or DELETE statement, the database marks an implicit savepoint (unavailable to you). If the statement fails, the database rolls back to the savepoint. Usually, just the failed SQL statement is rolled back, not the whole transaction. If the statement raises an unhandled exception, the host environment determines what is rolled back.

The database can also roll back single SQL statements to break deadlocks. The database signals an error to a participating transaction and rolls back the current statement in that transaction.

Before running a SQL statement, the database must parse it, that is, examine it to ensure it follows syntax rules and refers to valid schema objects. Errors detected while running a SQL statement cause a rollback, but errors detected while parsing the statement do not.

If you exit a stored subprogram with an unhandled exception, PL/SQL does not assign values to OUT parameters, and does not do any rollback.

SET TRANSACTION Statement

You use the SET TRANSACTION statement to begin a read-only or read-write transaction, establish an isolation level, or assign your current transaction to a specified rollback segment. Read-only transactions are useful for running multiple queries while other users update the same tables.

During a read-only transaction, all queries refer to the same snapshot of the database, providing a multi-table, multi-query, read-consistent view. Other users can continue to query or update data as usual. A commit or rollback ends the transaction.

In Example 6-43 a read-only transaction gather order totals for the day, the past week, and the past month. The totals are unaffected by other users updating the database during the transaction. The orders table is in the sample schema OE.

DECLARE
  daily_order_total    NUMBER(12,2);
  weekly_order_total   NUMBER(12,2); 
  monthly_order_total  NUMBER(12,2);
BEGIN
   COMMIT; -- end previous transaction
   SET TRANSACTION READ ONLY NAME 'Calculate Order Totals';

   SELECT SUM (order_total)
   INTO daily_order_total
   FROM orders
   WHERE order_date = SYSDATE;

   SELECT SUM (order_total)
   INTO weekly_order_total
   FROM orders
   WHERE order_date = SYSDATE - 7;

   SELECT SUM (order_total)
   INTO monthly_order_total
   FROM orders
   WHERE order_date = SYSDATE - 30;

   COMMIT; -- ends read-only transaction
END;
/

The SET TRANSACTION statement must be the first SQL statement in a read-only transaction and can only appear once in a transaction. If you set a transaction to READ ONLY, subsequent queries see only changes committed before the transaction began. The use of READ ONLY does not affect other users or transactions.

Only the SELECT, OPEN, FETCH, CLOSE, LOCK TABLE, COMMIT, and ROLLBACK statements are allowed in a read-only transaction. Queries cannot be FOR UPDATE.

Overriding Default Locking

By default, Oracle Database locks data structures automatically, which enables different applications to write to the same data structures without harming each other's data or coordinating with each other.

If you must have exclusive access to data during a transaction, you can override default locking with these SQL statements:

• LOCK TABLE, which explicitly locks entire tables.

• SELECT with the FOR UPDATE clause (SELECT FOR UPDATE) , which explicitly locks specific rows of a table.

Topics:

• LOCK TABLE Statement

• SELECT FOR UPDATE and FOR UPDATE Cursors

• Simulating CURRENT OF Clause with ROWID Pseudocolumn

DECLARE
  my_emp_id NUMBER(6);
  my_job_id VARCHAR2(10);
  my_sal    NUMBER(8,2);
  CURSOR c1 IS
    SELECT employee_id, job_id, salary
    FROM employees FOR UPDATE;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO my_emp_id, my_job_id, my_sal;
    IF my_job_id = 'SA_REP' THEN
      UPDATE employees
      SET salary = salary * 1.02
      WHERE CURRENT OF c1;
    END IF;
    EXIT WHEN c1%NOTFOUND;
  END LOOP;
END;
/

DECLARE
  CURSOR c1 IS
    SELECT last_name, department_name
    FROM employees, departments
    WHERE employees.department_id = departments.department_id 
    AND job_id = 'SA_MAN'
    FOR UPDATE OF salary;
BEGIN
  NULL;
END;
/

DECLARE
  CURSOR c1 IS
    SELECT * FROM employees
    FOR UPDATE OF salary;
  emp_rec  employees%ROWTYPE;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO emp_rec;  -- fails on second iteration
    EXIT WHEN c1%NOTFOUND;
    DBMS_OUTPUT.PUT_LINE (
      'emp_rec.employee_id = ' ||
      TO_CHAR(emp_rec.employee_id)
    );
    IF emp_rec.employee_id = 105 THEN
      UPDATE employees
      SET salary = salary * 1.05
      WHERE employee_id = 105;
    END IF;
    COMMIT;  -- releases locks
  END LOOP;
END;
/

emp_rec.employee_id = 100
DECLARE
*
ERROR at line 1:
ORA-01002: fetch out of sequence
ORA-06512: at line 9

DECLARE
  CURSOR c1 IS
    SELECT last_name, job_id, rowid
    FROM employees;  -- no FOR UPDATE clause
  my_lastname   employees.last_name%TYPE;
  my_jobid      employees.job_id%TYPE;
  my_rowid      UROWID;
BEGIN
  OPEN c1;
  LOOP
    FETCH c1 INTO my_lastname, my_jobid, my_rowid;
    EXIT WHEN c1%NOTFOUND;

    UPDATE employees
    SET salary = salary * 1.02
    WHERE rowid = my_rowid;  -- simulates WHERE CURRENT OF c1

    COMMIT;
  END LOOP;
  CLOSE c1;
END;
/

Advantages of Autonomous Transactions

Once started, an autonomous transaction is fully independent. It shares no locks, resources, or commit-dependencies with the main transaction. You can log events, increment retry counters, and so on, even if the main transaction rolls back.

Autonomous transactions help you build modular, reusable software components. You can encapsulate autonomous transactions in stored subprograms. A calling application needs not know whether operations done by that stored subprogram succeeded or failed.

Transaction Context

The main transaction shares its context with nested routines, but not with autonomous transactions. When one autonomous routine invokes another (or itself, recursively), the routines share no transaction context. When an autonomous routine invokes a nonautonomous routine, the routines share the same transaction context.

Transaction Visibility

Changes made by an autonomous transaction become visible to other transactions when the autonomous transaction commits. These changes become visible to the main transaction when it resumes, if its isolation level is set to READ COMMITTED (the default).

If you set the isolation level of the main transaction to SERIALIZABLE, changes made by its autonomous transactions are not visible to the main transaction when it resumes:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Declaring Autonomous Transactions

To declare an autonomous transaction, use the AUTONOMOUS_TRANSACTION pragma. For information about this pragma, see "AUTONOMOUS_TRANSACTION Pragma".

You cannot apply the AUTONOMOUS_TRANSACTION pragma to an entire package, but you can apply it to each subprogram in a package.

Example 6-48 marks a packaged function as autonomous.

CREATE OR REPLACE PACKAGE emp_actions AS  -- package specification
  FUNCTION raise_salary (emp_id NUMBER, sal_raise NUMBER)
  RETURN NUMBER;
END emp_actions;
/
CREATE OR REPLACE PACKAGE BODY emp_actions AS  -- package body
  -- code for function raise_salary
  FUNCTION raise_salary (emp_id NUMBER, sal_raise NUMBER)
  RETURN NUMBER IS
    PRAGMA AUTONOMOUS_TRANSACTION;
    new_sal NUMBER(8,2);
  BEGIN
    UPDATE employees SET salary =
      salary + sal_raise WHERE employee_id = emp_id;
    COMMIT;
    SELECT salary INTO new_sal FROM employees
      WHERE employee_id = emp_id;
    RETURN new_sal;
  END raise_salary;
END emp_actions;
/

Example 6-49 marks a standalone stored subprogram as autonomous.

CREATE OR REPLACE PROCEDURE lower_salary
   (emp_id NUMBER, amount NUMBER)
AS
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  UPDATE employees
  SET salary =  salary - amount
  WHERE employee_id = emp_id;

  COMMIT;
END lower_salary;
/

Example 6-50 marks a schema-level PL/SQL block as autonomous. (A nested PL/SQL block cannot be autonomous.)

DECLARE
  PRAGMA AUTONOMOUS_TRANSACTION;
  emp_id NUMBER(6)   := 200;
  amount NUMBER(6,2) := 200;
BEGIN
  UPDATE employees
  SET salary =  salary - amount
  WHERE employee_id = emp_id;

  COMMIT;
END;
/

Controlling Autonomous Transactions

The first SQL statement in an autonomous routine begins a transaction. When one transaction ends, the next SQL statement begins another transaction. All SQL statements run since the last commit or rollback comprise the current transaction. To control autonomous transactions, use these statements, which apply only to the current (active) transaction:

• COMMIT

• ROLLBACK [TO savepoint_name]

• SAVEPOINT savepoint_name

• SET TRANSACTION

Topics:

• Entering and Exiting

• Committing and Rolling Back

• Savepoints

• Avoiding Errors with Autonomous Transactions

Autonomous Triggers

A database trigger can do the following only if it is autonomous:

• Run Transaction Control Language (TCL) statements

• Use native dynamic SQL to run data definition language (DDL) statements

  For information about native dynamic SQL, see "Native Dynamic SQL".

One use of database triggers is to log events transparently—for example, to log all inserts into a table, even those that roll back. In Example 6-51, whenever a row is inserted into the EMPLOYEES table, a trigger inserts the same row into a log table. Because the trigger is autonomous, it can commit changes to the log table regardless of whether they are committed to the main table.

-- Log table:

DROP TABLE log;
CREATE TABLE log (
  log_id  NUMBER(6),
  up_date DATE,
  new_sal NUMBER(8,2),
  old_sal NUMBER(8,2)
);

-- Autonomous trigger on employees table:

CREATE OR REPLACE TRIGGER log_sal
  BEFORE UPDATE OF salary ON employees FOR EACH ROW
DECLARE
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  INSERT INTO log (
    log_id,
    up_date,
    new_sal,
    old_sal
  )
  VALUES (
    :old.employee_id,
    SYSDATE,
    :new.salary,
    :old.salary
  );
  COMMIT;
END;
/
UPDATE employees
SET salary = salary * 1.05
WHERE employee_id = 115;

COMMIT;

UPDATE employees
SET salary = salary * 1.05
WHERE employee_id = 116;

ROLLBACK;

-- Show that both committed and rolled-back updates
-- add rows to log table

SELECT * FROM log
WHERE log_id = 115 OR log_id = 116;

LOG_ID UP_DATE      NEW_SAL    OLD_SAL
---------- --------- ---------- ----------
       115 25-AUG-09       3255       3100
       116 25-AUG-09       3045       2900
 
2 rows selected.

In Example 6-52, an autonomous trigger uses native dynamic SQL (an EXECUTE IMMEDIATE statement) to drop a temporary table after a row is inserted into the table log.

DROP TABLE temp;
CREATE TABLE temp (
  temp_id NUMBER(6),
  up_date DATE
);

CREATE OR REPLACE TRIGGER drop_temp_table
  AFTER INSERT ON log
DECLARE 
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  EXECUTE IMMEDIATE 'DROP TABLE temp';
  COMMIT;
END;
/
-- Show how trigger works
SELECT * FROM temp;
 

no rows selected

INSERT INTO log (log_id, up_date, new_sal, old_sal)
VALUES (999, SYSDATE, 5000, 4500);
 
1 row created.
 
SELECT * FROM temp;
 

SELECT * FROM temp
              *
ERROR at line 1:
ORA-00942: table or view does not exist

For general information about triggers, see Chapter 9, "PL/SQL Triggers."

Invoking Autonomous Functions from SQL

A function invoked from SQL statements must obey rules meant to control side effects (for details, see "Subprogram Side Effects"). To check for violations of the rules, use the RESTRICT_REFERENCES pragma. This pragma asserts that a function does not read or write database tables or package variables. For more information about this pragma, see Oracle Database Advanced Application Developer's Guide.

By definition, autonomous routines never violate the rules read no database state (RNDS) and write no database state (WNDS).

The packaged function log_msg in Example 6-53 is autonomous. Therefore, when the query invokes the function, the function inserts a message into database table debug_output without violating the rule write no database state.

DROP TABLE debug_output;
CREATE TABLE debug_output (message VARCHAR2(200));
 
CREATE OR REPLACE PACKAGE debugging AS
  FUNCTION log_msg (msg VARCHAR2) RETURN VARCHAR2;
  PRAGMA RESTRICT_REFERENCES(log_msg, WNDS, RNDS);
END debugging;
/
CREATE OR REPLACE PACKAGE BODY debugging AS
  FUNCTION log_msg (msg VARCHAR2) RETURN VARCHAR2 IS
    PRAGMA AUTONOMOUS_TRANSACTION;
  BEGIN
    /* This insert does not violate the constraint WNDS
       because this is an autonomous routine */
  
  INSERT INTO debug_output (message)
  VALUES (msg);
  
  COMMIT;
  RETURN msg;
  END;
END debugging;
/
-- Invoke packaged function from query
DECLARE
  my_emp_id    NUMBER(6);
  my_last_name VARCHAR2(25);
  my_count     NUMBER;
BEGIN
  my_emp_id := 120;
 
  SELECT debugging.log_msg(last_name)
  INTO my_last_name
  FROM employees
  WHERE employee_id = my_emp_id;
 
  /* Even if you roll back in this scope,
     the insert into 'debug_output' remains committed,
     because it is part of an autonomous transaction. */
 
  ROLLBACK;
END;
/