10 Using Dynamic SQL

Method 1

This method lets your program accept or build a dynamic SQL statement, then immediately execute it using the EXECUTE IMMEDIATE command. The SQL statement must not be a query (SELECT statement) and must not contain any placeholders for input host variables. For example, the following host strings qualify:

'DELETE FROM EMP WHERE DEPTNO = 20'
'GRANT SELECT ON EMP TO scott'

With Method 1, the SQL statement is parsed every time it is executed (unless you specify HOLD_CURSOR=YES).

Method 2

This method lets your program accept or build a dynamic SQL statement, then process it using the PREPARE and EXECUTE commands .The SQL statement must not be a query. The number of placeholders for input host variables and the datatypes of the input host variables must be known at precompile time. For example, the following host strings fall into this category:

'INSERT INTO EMP (ENAME, JOB) VALUES (:emp_name, :job_title)'
'DELETE FROM EMP WHERE EMPNO = :emp_number'

With Method 2, the SQL statement is parsed just once (unless you specify RELEASE_CURSOR=YES), but it can be executed many times with different values for the host variables. SQL data definition statements such as CREATE are executed when they are PREPAREd.

Method 3

This method lets your program accept or build a dynamic query, then process it using the PREPARE command with the DECLARE, OPEN, FETCH, and CLOSE cursor commands. The number of select-list items, the number of placeholders for input host variables, and the datatypes of the input host variables must be known at precompile time. For example, the following host strings qualify:

'SELECT DEPTNO, MIN(SAL), MAX(SAL) FROM EMP GROUP BY DEPTNO'
'SELECT ENAME, EMPNO FROM EMP WHERE DEPTNO = :dept_number'

Method 4

This method lets your program accept or build a dynamic SQL statement, then process it using descriptors (discussed in "Using Method 4"). The number of select-list items, the number of placeholders for input host variables, and the datatypes of the input host variables can be unknown until run time. For example, the following host strings fall into this category:

'INSERT INTO EMP (<unknown>) VALUES (<unknown>)'
'SELECT <unknown> FROM EMP WHERE DEPTNO = 20'

Method 4 is required for dynamic SQL statements that contain an unknown number of select-list items or input host variables.

Guidelines

With all four methods, you must store the dynamic SQL statement in a character string, which must be a host variable or quoted literal. When you store the SQL statement in the string, omit the keywords EXEC SQL and the statement terminator.

With Methods 2 and 3, the number of placeholders for input host variables and the datatypes of the input host variables must be known at precompile time.

Each succeeding method imposes fewer constraints on your application, but is more difficult to code. As a rule, use the simplest method you can. However, if a dynamic SQL statement is to be executed repeatedly by Method 1, use Method 2 instead to avoid reparsing for each execution.

Method 4 provides maximum flexibility, but requires complex coding and a full understanding of dynamic SQL concepts. In general, use Method 4 only if you cannot use Methods 1, 2, or 3. The decision logic in Figure 10-1 will help you choose the correct method.

Avoiding Common Errors

If you use a character array to store the dynamic SQL statement, blank-pad the array before storing the SQL statement. That way, you clear extraneous characters. This is especially important when you reuse the array for different SQL statements. As a rule, always initialize (or reinitialize) the host string before storing the SQL statement.

Do not null-terminate the host string. Oracle does not recognize the null terminator as an end-of-string sentinel. Instead, Oracle treats it as part of the SQL statement.

If you use a VARCHAR variable to store the dynamic SQL statement, make sure the length of the VARCHAR is set (or reset) correctly before you execute the PREPARE or EXECUTE IMMEDIATE statement.

EXECUTE resets the SQLWARN warning flags in the SQLCA. So, to catch mistakes such as an unconditional update (caused by omitting a WHERE clause), check the SQLWARN flags after executing the PREPARE statement but before executing the EXECUTE statement.

Figure 10-1 Choosing the Right Method

Description of "Figure 10-1 Choosing the Right Method"

The EXECUTE IMMEDIATE Statement

Method 1 parses, then immediately executes the SQL statement using the EXECUTE IMMEDIATE command. The command is followed by a character string (host variable or literal) containing the SQL statement to be executed, which cannot be a query.

The syntax of the EXECUTE IMMEDIATE statement follows:

EXEC SQL EXECUTE IMMEDIATE { :host_string | string_literal };

In the following example, you use the host variable sql_stmt to store SQL statements input by the user:

EXEC SQL BEGIN DECLARE SECTION;
 ...
 sql_stmt CHARACTER(120);
EXEC SQL END DECLARE SECTION;
...
LOOP
 display 'Enter SQL statement: ';
 read sql_stmt;
 IF sql_stmt is empty THEN
 exit loop;
 ENDIF;
 -- sql_stmt now contains the text of a SQL statement
 EXEC SQL EXECUTE IMMEDIATE :sql_stmt;
ENDLOOP;

You can also use string literals, as the following example shows:

EXEC SQL EXECUTE IMMEDIATE 'REVOKE RESOURCE FROM MILLER';

Because EXECUTE IMMEDIATE parses the input SQL statement before every execution, Method 1 is best for statements that are executed only once. Data definition statements usually fall into this category.

An Example

The following program prompts the user for a search condition to be used in the WHERE clause of an UPDATE statement, then executes the statement using Method 1:

EXEC SQL BEGIN DECLARE SECTION;
 username CHARACTER(20);
 password CHARACTER(20);
 update_stmt CHARACTER(120);
EXEC SQL END DECLARE SECTION;
 search_cond CHARACTER(40);
EXEC SQL INCLUDE SQLCA;
display 'Username? ';
read username;
display 'Password? ';
read password;
EXEC SQL WHENEVER SQLERROR GOTO sql_error;
EXEC SQL CONNECT :username IDENTIFIED BY :password;
display 'Connected to Oracle';
set update_stmt = 'UPDATE EMP SET COMM = 500 WHERE ';
display 'Enter a search condition for the following statement:';
display update_stmt;
read search_cond;
concatenate update_stmt, search_cond;
EXEC SQL EXECUTE IMMEDIATE :update_stmt;
EXEC SQL COMMIT WORK RELEASE;
exit program;
sql_error:
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 EXEC SQL ROLLBACK WORK RELEASE;
 display 'Processing error';
 exit program with an error;

The USING Clause

When the SQL statement is EXECUTEd, input host variables in the USING clause replace corresponding placeholders in the PREPAREd dynamic SQL statement.

Every placeholder in the PREPAREd dynamic SQL statement must correspond to a host variable in the USING clause. So, if the same placeholder appears two or more times in the PREPAREd statement, each appearance must correspond to a host variable in the USING clause. If one of the host variables in the USING clause is an array, all must be arrays.

The names of the placeholders need not match the names of the host variables. However, the order of the placeholders in the PREPAREd dynamic SQL statement must match the order of corresponding host variables in the USING clause.

To specify nulls, you can associate indicator variables with host variables in the USING clause. For more information, refer to "Using Indicator Variables".

An Example

The following program prompts the user for a search condition to be used in the WHERE clause of an UPDATE statement, then prepares and executes the statement using Method 2. Notice that the SET clause of the UPDATE statement contains a placeholder (c).

EXEC SQL BEGIN DECLARE SECTION;
 username CHARACTER(20);
 password CHARACTER(20);
 sql_stmt CHARACTER(80);
 empno INTEGER VALUE 1234;
 deptno1 INTEGER VALUE 97;
 deptno2 INTEGER VALUE 99;
EXEC SQL END DECLARE SECTION;
EXEC SQL INCLUDE SQLCA;
EXEC ORACLE OPTION (ORACA=YES);
EXEC SQL WHENEVER SQLERROR GOTO sql_error;
display 'Username? ';
read username;
display 'Password? ';
read password;
EXEC SQL CONNECT :username IDENTIFIED BY :password;
display 'Connected to Oracle';
set sql_stmt = 
 'INSERT INTO EMP (EMPNO, DEPTNO) VALUES (:v1, :v2)';
display "V1 = ", empno, "V2 = ", deptno1;
EXEC SQL PREPARE S FROM :sql_stmt;
EXEC SQL EXECUTE S USING :empno, :deptno1;
set empno = empno + 1;
display "V1 = ", empno, "V2 = ", deptno2;
EXEC SQL EXECUTE S USING :empno, :deptno2;
set sql_stmt = 
 'DELETE FROM EMP WHERE DEPTNO = :v1 OR DEPTNO = :v2")';
display "V1 = ", deptno1, "V2 = ", deptno2;
EXEC SQL PREPARE S FROM :sql_stmt;
EXEC SQL EXECUTE S USING :deptno1, :deptno2;
EXEC SQL COMMIT WORK RELEASE;
exit program;
sql_error:
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 display 'Processing error';
 EXEC SQL ROLLBACK WORK RELEASE;
 exit program with an error;

PREPARE

PREPARE parses the dynamic SQL statement and gives it a name. In the following example, PREPARE parses the query stored in the character string select_stmt and gives it the name sql_stmt:

set select_stmt = 'SELECT MGR, JOB FROM EMP WHERE SAL < :salary';
EXEC SQL PREPARE sql_stmt FROM :select_stmt;

Commonly, the query WHERE clause is input from a terminal at run time or is generated by the application.

The identifier sql_stmt is not a host or program variable, but must be unique. It designates a particular dynamic SQL statement.

DECLARE

DECLARE defines a cursor by giving it a name and associating it with a specific query. The cursor declaration is local to its precompilation unit. Continuing our example, DECLARE defines a cursor named emp_cursor and associates it with sql_stmt, as follows:

EXEC SQL DECLARE emp_cursor CURSOR FOR sql_stmt;

The identifiers sql_stmt and emp_cursor are not host or program variables, but must be unique. If you declare two cursors using the same statement name, the precompiler considers the two cursor names synonymous. For example, if you execute the statements

EXEC SQL PREPARE sql_stmt FROM :select_stmt;
EXEC SQL DECLARE emp_cursor FOR sql_stmt;
EXEC SQL PREPARE sql_stmt FROM :delete_stmt;
EXEC SQL DECLARE dept_cursor FOR sql_stmt;

when you OPEN emp_cursor, you will process the dynamic SQL statement stored in delete_stmt, not the one stored in select_stmt.

OPEN

OPEN allocates an Oracle cursor, binds input host variables, and executes the query, identifying its active set. OPEN also positions the cursor on the first row in the active set and zeroes the rows-processed count kept by the third element of SQLERRD in the SQLCA. Input host variables in the USING clause replace corresponding placeholders in the PREPAREd dynamic SQL statement.

In our example, OPEN allocates emp_cursor and assigns the host variable salary to the WHERE clause, as follows:

EXEC SQL OPEN emp_cursor USING :salary;

FETCH

FETCH returns a row from the active set, assigns column values in the select list to corresponding host variables in the INTO clause, and advances the cursor to the next row. When no more rows are found, FETCH returns the "no data found" Oracle error code to SQLCODE in the SQLCA.

In our example, FETCH returns a row from the active set and assigns the values of columns MGR and JOB to host variables mgr_number and job_title, as follows:

EXEC SQL FETCH emp_cursor INTO :mgr_number, :job_title;

CLOSE

CLOSE disables the cursor. Once you CLOSE a cursor, you can no longer FETCH from it. In our example, the CLOSE statement disables emp_cursor, as follows:

EXEC SQL CLOSE emp_cursor;

An Example

The following program prompts the user for a search condition to be used in the WHERE clause of a query, then prepares and executes the query using Method 3.

EXEC SQL BEGIN DECLARE SECTION;
 username CHARACTER(20);
 password CHARACTER(20);
 dept_number INTEGER;
 emp_name CHARACTER(10);
 salary REAL;
 select_stmt CHARACTER(120);
EXEC SQL END DECLARE SECTION;
 search_cond CHARACTER(40);
EXEC SQL INCLUDE SQLCA;
display 'Username? ';
read username;
display 'Password? ';
read password;
EXEC SQL WHENEVER SQLERROR GOTO sql_error;
EXEC SQL CONNECT :username IDENTIFIED BY :password;
display 'Connected to Oracle';
set select_stmt = 'SELECT ENAME,SAL FROM EMP WHERE ';
display 'Enter a search condition for the following statement:';
display select_stmt;
read search_cond;
concatenate select_stmt, search_cond;
EXEC SQL PREPARE sql_stmt FROM :select_stmt;
EXEC SQL DECLARE emp_cursor CURSOR FOR sql_stmt;
EXEC SQL OPEN emp_cursor;
EXEC SQL WHENEVER NOT FOUND GOTO no_more;
display 'Employee Salary';
display '-------- ------';
LOOP
 EXEC SQL FETCH emp_cursor INTO :emp_name, :salary;
 display emp_name, salary;
ENDLOOP;
no_more:
 EXEC SQL CLOSE emp_cursor;
 EXEC SQL COMMIT WORK RELEASE;
 exit program;
sql_error:
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 EXEC SQL ROLLBACK WORK RELEASE;
 exit program with an error;

Need for the SQLDA

To process this kind of dynamic query, your program must issue the DESCRIBE SELECT LIST command and declare a data structure called the SQL Descriptor Area (SQLDA). Because it holds descriptions of columns in the query select list, this structure is also called a select descriptor.

Likewise, if a dynamic SQL statement contains an unknown number of placeholders for input host variables, the host-variable list cannot be established at precompile time by the USING clause.

To process the dynamic SQL statement, your program must issue the DESCRIBE BIND VARIABLES command and declare another kind of SQLDA called a bind descriptor to hold descriptions of the placeholders for the input host variables. (Input host variables are also called bind variables.)

If your program has more than one active SQL statement (it might have OPENed two or more cursors, for example), each statement must have its own SQLDA(s). However, non-concurrent cursors can reuse SQLDAs. There is no set limit on the number of SQLDAs in a program.

The DESCRIBE Statement

DESCRIBE initializes a descriptor to hold descriptions of select-list items or input host variables.

If you supply a select descriptor, the DESCRIBE SELECT LIST statement examines each select-list item in a PREPAREd dynamic query to determine its name, datatype, constraints, length, scale, and precision. It then stores this information in the select descriptor.

If you supply a bind descriptor, the DESCRIBE BIND VARIABLES statement examines each placeholder in a PREPAREd dynamic SQL statement to determine its name, length, and the datatype of its associated input host variable. It then stores this information in the bind descriptor for your use. For example, you might use placeholder names to prompt the user for the values of input host variables.

What Is a SQLDA?

A SQLDA is a host-program data structure that holds descriptions of select-list items or input host variables.

SQLDA variables are not defined in the Declare Section.

Though SQLDAs differ among host languages, a generic select SQLDA contains the following information about a query select list:

• Maximum number of columns that can be described

• Actual number of columns found by describe

• Addresses of buffers to store column values

• Lengths of column values

• Datatypes of column values

• addresses of indicator-variable values

• Addresses of buffers to store column names

• Sizes of buffers to store column names

• Current lengths of column names

A generic bind SQLDA contains the following information about the input host variables in a SQL statement:

• Maximum number of placeholders that can be described

• Actual number of placeholders found by describe

• Addresses of input host variables

• Lengths of input host variables

• Datatypes of input host variables

• Addresses of indicator variables

• Addresses of buffers to store placeholder names

• Sizes of buffers to store placeholder names

• Current lengths of placeholder names

• Addresses of buffers to store indicator-variable names

• Sizes of buffers to store indicator-variable names

• Current lengths of indicator-variable names

To see the SQLDA structure and variable names in a particular host language, refer to your host-language supplement.

Implementing Method 4

With Method 4, you generally use the following sequence of embedded SQL statements:

EXEC SQL PREPARE statement_name
 FROM { :host_string | string_literal };
EXEC SQL DECLARE cursor_name CURSOR FOR statement_name;
EXEC SQL DESCRIBE BIND VARIABLES FOR statement_name
 INTO bind_descriptor_name;
EXEC SQL OPEN cursor_name
 [USING DESCRIPTOR bind_descriptor_name];
EXEC SQL DESCRIBE [SELECT LIST FOR] statement_name
 INTO select_descriptor_name;
EXEC SQL FETCH cursor_name
 USING DESCRIPTOR select_descriptor_name;
EXEC SQL CLOSE cursor_name;

Select and bind descriptors need not work in tandem. If the number of columns in a query select list is known, but the number of placeholders for input host variables is unknown, you can use the Method 4 OPEN statement with the following Method 3 FETCH statement:

EXEC SQL FETCH emp_cursor INTO host_variable_list;

Conversely, if the number of placeholders for input host variables is known, but the number of columns in the select list is unknown, you can use the following Method 3 OPEN statement with the Method 4 FETCH statement:

EXEC SQL OPEN cursor_name [USING host_variable_list];

Note that EXECUTE can be used for nonqueries with Method 4.

To learn how these statements allow your program to process dynamic SQL statements using descriptors, see your host-language supplement.

Usage of Host Arrays

Usage of host arrays in static and dynamic SQL is similar. For example, to use input host arrays with dynamic SQL Method 2, use the syntax

EXEC SQL EXECUTE statement_name USING host_array_list;

where host_array_list contains one or more host arrays. With Method 3, use the following syntax:

OPEN cursor_name USING host_array_list;

To use output host arrays with Method 3, use the following syntax:

FETCH cursor_name INTO host_array_list;

With Method 4, you must use the optional FOR clause to tell Oracle the size of your input or output host array. To learn how this is done, see your host-language supplement.

With Method 1

If the PL/SQL block contains no host variables, you can use Method 1 to EXECUTE the PL/SQL string in the usual way.

With Method 2

If the PL/SQL block contains a known number of input and output host variables, you can use Method 2 to PREPARE and EXECUTE the PL/SQL string in the usual way.

You must put all host variables in the USING clause. When the PL/SQL string is EXECUTEd, host variables in the USING clause replace corresponding placeholders in the PREPAREd string. Though the precompiler treats all PL/SQL host variables as input host variables, values are assigned correctly. Input (program) values are assigned to input host variables, and output (column) values are assigned to output host variables.

Every placeholder in the PREPAREd PL/SQL string must correspond to a host variable in the USING clause. So, if the same placeholder appears two or more times in the PREPAREd string, each appearance must correspond to a host variable in the USING clause.

With Method 3

Methods 2 and 3 are the same except that Method 3 allows FETCHing. Since you cannot FETCH from a PL/SQL block, use Method 2 instead.

With Method 4

If the PL/SQL block contains an unknown number of input or output host variables, you must use Method 4.

To use Method 4, you set up one bind descriptor for all the input and output host variables. Executing DESCRIBE BIND VARIABLES stores information about input and output host variables in the bind descriptor. Because the precompiler treats all PL/SQL host variables as input host variables, executing DESCRIBE SELECT LIST has no effect.

The use of bind descriptors with Method 4 is detailed in your host-language supplement.

Caution

Do not use ANSI-style comments (- - ...) in a PL/SQL block that will be processed dynamically because end-of-line characters are ignored. As a result, ANSI-style comments extend to the end of the block, not just to the end of a line. Instead, use C-style comments (/* ... */).