13 Oracle 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.

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, but can be executed many times with different values for the host variables. SQL data definition statements such as CREATE and GRANT 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. 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 will 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 13-1 will help you choose the right method.

Avoiding Common Errors

If you precompile using the command-line option DBMS=V6_CHAR, 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 re-initialize) 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 precompile with the command-line option DBMS=V8, make sure that the string is null terminated before you execute the PREPARE or EXECUTE IMMEDIATE statement.

Regardless of the value of DBMS, 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.

Figure 13-1 Choosing the Right Method

Description of "Figure 13-1 Choosing the Right Method"

Example Program: Dynamic SQL Method 1

The following program uses dynamic SQL Method 1 to create a table, insert a row, commit the insert, then drop the table. This program is available on-line in your demo directory in the file sample6.pc.

/*
 *  sample6.pc: Dynamic SQL Method 1
 *
 *  This program uses dynamic SQL Method 1 to create a table,
 *  insert a row, commit the insert, then drop the table.
 */

#include <stdio.h>
#include <string.h>

/* Include the SQL Communications Area, a structure through
 * which ORACLE makes runtime status information such as error
 * codes, warning flags, and diagnostic text available to the
 * program.
 */
#include <sqlca.h>

/* Include the ORACLE Communications Area, a structure through
 * which ORACLE makes additional runtime status information
 * available to the program.
 */
#include <oraca.h>

/* The ORACA=YES option must be specified to enable you
 * to use the ORACA.
 */

EXEC ORACLE OPTION (ORACA=YES);

/* Specifying the RELEASE_CURSOR=YES option instructs Pro*C
 * to release resources associated with embedded SQL
 * statements after they are executed.  This ensures that
 * ORACLE does not keep parse locks on tables after data
 * manipulation operations, so that subsequent data definition
 * operations on those tables do not result in a parse-lock
 * error.
 */

EXEC ORACLE OPTION (RELEASE_CURSOR=YES);

void dyn_error();


main()
{
/* Declare the program host variables. */
    char    *username = "SCOTT";
    char    *password = "TIGER";
    char    *dynstmt1;
    char     dynstmt2[10];
    VARCHAR  dynstmt3[80];

/* Call routine dyn_error() if an ORACLE error occurs. */

    EXEC SQL WHENEVER SQLERROR DO dyn_error("Oracle error:");

/* Save text of current SQL statement in the ORACA if an
 * error occurs.
 */
    oraca.orastxtf = ORASTFERR;

/* Connect to Oracle. */

    EXEC SQL CONNECT :username IDENTIFIED BY :password;
    puts("\nConnected to ORACLE.\n");

/* Execute a string literal to create the table.  This
 * usage is actually not dynamic because the program does
 * not determine the SQL statement at run time.
 */
    puts("CREATE TABLE dyn1 (col1 VARCHAR2(4))");

    EXEC SQL EXECUTE IMMEDIATE
         "CREATE TABLE dyn1 (col1 VARCHAR2(4))";

/* Execute a string to insert a row.  The string must
 * be null-terminated.  This usage is dynamic because the
 * SQL statement is a string variable whose contents the
 * program can determine at run time.
 */
    dynstmt1 = "INSERT INTO DYN1 values ('TEST')";
    puts(dynstmt1);

    EXEC SQL EXECUTE IMMEDIATE :dynstmt1;

/* Execute a SQL statement in a string to commit the insert.
 * Pad the unused trailing portion of the array with spaces.
 * Do NOT null-terminate it.
 */
    strncpy(dynstmt2, "COMMIT    ", 10);
    printf("%.10s\n", dynstmt2);

    EXEC SQL EXECUTE IMMEDIATE :dynstmt2;

/* Execute a VARCHAR to drop the table.  Set the .len field
 * to the length of the .arr field.
 */
    strcpy(dynstmt3.arr, "DROP TABLE DYN1");
    dynstmt3.len = strlen(dynstmt3.arr);
    puts((char *) dynstmt3.arr);

    EXEC SQL EXECUTE IMMEDIATE :dynstmt3;

/* Commit any outstanding changes and disconnect from Oracle. */
    EXEC SQL COMMIT RELEASE;

    puts("\nHave a good day!\n");

    return 0;
}


void
dyn_error(msg)
char *msg;
{
/* This is the Oracle error handler.
 * Print diagnostic text containing the error message,
 * current SQL statement, and location of error.
 */
    printf("\n%.*s\n",
       sqlca.sqlerrm.sqlerrml, sqlca.sqlerrm.sqlerrmc);
    printf("in \"%.*s...\'\n",
        oraca.orastxt.orastxtl, oraca.orastxt.orastxtc);
    printf("on line %d of %.*s.\n\n",
        oraca.oraslnr, oraca.orasfnm.orasfnml,
        oraca.orasfnm.orasfnmc);

/* Disable Oracle error checking to avoid an infinite loop
 * should another error occur within this routine as a 
 * result of the rollback.
 */
    EXEC SQL WHENEVER SQLERROR CONTINUE;

/* Roll back any pending changes and disconnect from Oracle. */
    EXEC SQL ROLLBACK RELEASE;

    exit(1);
}

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 different 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.

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.

If one of the host variables in the USING clause is an array, all must be arrays.

To specify NULLs, you can associate indicator variables with host variables in the USING clause.

Example Program: Dynamic SQL Method 2

The following program uses dynamic SQL Method 2 to insert two rows into the EMP table, then delete them. This program is available on-line in your demo directory, in the file sample7.pc.

/*
 *  sample7.pc: Dynamic SQL Method 2
 *
 *  This program uses dynamic SQL Method 2 to insert two rows into
 *  the EMP table, then delete them.
 */

#include <stdio.h>
#include <string.h>

#define USERNAME "SCOTT"
#define PASSWORD "TIGER"

/* Include the SQL Communications Area, a structure through
 * which ORACLE makes runtime status information such as error
 * codes, warning flags, and diagnostic text available to the
 * program.
 */
#include <sqlca.h>

/* Include the ORACLE Communications Area, a structure through
 * which ORACLE makes additional runtime status information
 * available to the program.
 */
#include <oraca.h>

/* The ORACA=YES option must be specified to enable use of
 * the ORACA.
 */
EXEC ORACLE OPTION (ORACA=YES);

char    *username = USERNAME;
char    *password = PASSWORD;
VARCHAR  dynstmt[80];
int      empno   = 1234;
int      deptno1 = 97;
int      deptno2 = 99;


/* Handle SQL runtime errors. */
void dyn_error();


main()
{
/* Call dyn_error() whenever an error occurs
 * processing an embedded SQL statement.
 */
    EXEC SQL WHENEVER SQLERROR DO dyn_error("Oracle error");

/* Save text of current SQL statement in the ORACA if an
 * error occurs.
 */
    oraca.orastxtf = ORASTFERR;

/* Connect to Oracle. */

    EXEC SQL CONNECT :username IDENTIFIED BY :password;
    puts("\nConnected to Oracle.\n");

/* Assign a SQL statement to the VARCHAR dynstmt.  Both
 * the array and the length parts must be set properly.
 * Note that the statement contains two host-variable
 * placeholders, v1 and v2, for which actual input
 * host variables must be supplied at EXECUTE time.
 */
    strcpy(dynstmt.arr,
        "INSERT INTO EMP (EMPNO, DEPTNO) VALUES (:v1, :v2)");
    dynstmt.len = strlen(dynstmt.arr);

/* Display the SQL statement and its current input host
 * variables.
 */
    puts((char *) dynstmt.arr);
    printf("   v1 = %d,  v2 = %d\n", empno, deptno1);

/* The PREPARE statement associates a statement name with
 * a string containing a SQL statement.  The statement name
 * is a SQL identifier, not a host variable, and therefore
 * does not appear in the Declare Section.

 * A single statement name can be PREPAREd more than once,
 * optionally FROM a different string variable.
 */
    EXEC SQL PREPARE S FROM :dynstmt;

/* The EXECUTE statement executes a PREPAREd SQL statement
 * USING the specified input host variables, which are
 * substituted positionally for placeholders in the
 * PREPAREd statement.  For each occurrence of a
 * placeholder in the statement there must be a variable
 * in the USING clause.  That is, if a placeholder occurs
 * multiple times in the statement, the corresponding
 * variable must appear multiple times in the USING clause.
 * The USING clause can be omitted only if the statement
 * contains no placeholders.
 *
 * A single PREPAREd statement can be EXECUTEd more
 * than once, optionally USING different input host
 * variables.
 */
    EXEC SQL EXECUTE S USING :empno, :deptno1;

/* Increment empno and display new input host variables. */

    empno++;
    printf("   v1 = %d,  v2 = %d\n", empno, deptno2);

/* ReEXECUTE S to insert the new value of empno and a
 * different input host variable, deptno2.
 * A rePREPARE is unnecessary.
 */
    EXEC SQL EXECUTE S USING :empno, :deptno2;

/* Assign a new value to dynstmt. */

    strcpy(dynstmt.arr,
        "DELETE FROM EMP WHERE DEPTNO = :v1 OR DEPTNO = :v2");
    dynstmt.len = strlen(dynstmt.arr);

/* Display the new SQL statement and its current input host
 * variables.
 */
    puts((char *) dynstmt.arr);
    printf("   v1 = %d,    v2 = %d\n", deptno1, deptno2);

/* RePREPARE S FROM the new dynstmt. */

    EXEC SQL PREPARE S FROM :dynstmt;

/* EXECUTE the new S to delete the two rows previously
 * inserted.
 */
    EXEC SQL EXECUTE S USING :deptno1, :deptno2;

/* Commit any pending changes and disconnect from Oracle. */

    EXEC SQL COMMIT RELEASE;
    puts("\nHave a good day!\n");
    exit(0);
}


void 
dyn_error(msg)
char *msg;
{
/* This is the ORACLE error handler.
 * Print diagnostic text containing error message,
 * current SQL statement, and location of error.
 */
    printf("\n%s", msg);
    printf("\n%.*s\n",
        sqlca.sqlerrm.sqlerrml, sqlca.sqlerrm.sqlerrmc);
    printf("in \"%.*s...\"\n",
        oraca.orastxt.orastxtl, oraca.orastxt.orastxtc);
    printf("on line %d of %.*s.\n\n",
        oraca.oraslnr, oraca.orasfnm.orasfnml,
        oraca.orasfnm.orasfnmc);

/* Disable ORACLE error checking to avoid an infinite loop
 * should another error occur within this routine.
 */
    EXEC SQL WHENEVER SQLERROR CONTINUE;

/* Roll back any pending changes and 
 * disconnect from Oracle.
 */
    EXEC SQL ROLLBACK RELEASE;
    exit(1);
}

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:

char select_stmt[132] =
     "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.

The following statement is correct also:

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

The following prepare statement, which uses the '%' wildcard, is correct also:

EXEC SQL PREPARE S FROM select ename FROM test WHERE ename LIKE 'SMIT%';

DECLARE

DECLARE defines a cursor by giving it a name and associating it with a specific query. 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.

We can define a scrollable cursor named emp_cursor and associate it with sql_stmt as follows:

EXEC SQL DECLARE emp_cursor SCROLL CURSOR FOR sql_stmt;

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. If there are no more rows, FETCH returns the "no data found" Oracle error code to sqlca.sqlcode.

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; 

If the cursor is declared in SCROLL mode, you can then use the various FETCH orientation modes to randomly access the result set.

CLOSE

CLOSE disables the cursor. Once you CLOSE a cursor, you can no longer FETCH from it.

In our example, CLOSE disables emp_cursor, as follows:

EXEC SQL CLOSE emp_cursor; 

Example Program: Dynamic SQL Method 3

The following program uses dynamic SQL Method 3 to retrieve the names of all employees in a given department from the EMP table. This program is available on-line in your demo directory, in the file sample8.pc

/*
 *  sample8.pc:  Dynamic SQL Method 3
 *
 *  This program uses dynamic SQL Method 3 to retrieve the names
 *  of all employees in a given department from the EMP table.
 */

#include <stdio.h>
#include <string.h>

#define USERNAME "SCOTT"
#define PASSWORD "TIGER"

/* Include the SQL Communications Area, a structure through
 * which ORACLE makes runtime status information such as error
 * codes, warning flags, and diagnostic text available to the
 * program. Also include the ORACA.
 */
#include <sqlca.h>
#include <oraca.h>

/* The ORACA=YES option must be specified to enable use of
 * the ORACA.
 */
EXEC ORACLE OPTION (ORACA=YES);

char    *username = USERNAME;
char    *password = PASSWORD;
VARCHAR  dynstmt[80];
VARCHAR  ename[10];
int      deptno = 10;

void dyn_error();


main()
{
/* Call dyn_error() function on any error in
 * an embedded SQL statement.
 */
    EXEC SQL WHENEVER SQLERROR DO dyn_error("Oracle error");

/* Save text of SQL current statement in the ORACA if an
 * error occurs.
 */
    oraca.orastxtf = ORASTFERR;

/* Connect to Oracle. */

    EXEC SQL CONNECT :username IDENTIFIED BY :password;
    puts("\nConnected to Oracle.\n");

/* Assign a SQL query to the VARCHAR dynstmt.  Both the
 * array and the length parts must be set properly.  Note
 * that the query contains one host-variable placeholder,
 * v1, for which an actual input host variable must be
 * supplied at OPEN time.
 */
    strcpy(dynstmt.arr,
        "SELECT ename FROM emp WHERE deptno = :v1");
    dynstmt.len = strlen(dynstmt.arr);

/* Display the SQL statement and its current input host
 * variable.
 */
    puts((char *) dynstmt.arr);
    printf("   v1 = %d\n", deptno);
    printf("\nEmployee\n");
    printf("--------\n");

/* The PREPARE statement associates a statement name with
 * a string containing a SELECT statement.  The statement
 * name is a SQL identifier, not a host variable, and
 * therefore does not appear in the Declare Section.

 * A single statement name can be PREPAREd more than once,
 * optionally FROM a different string variable.
 */
    EXEC SQL PREPARE S FROM :dynstmt;

/* The DECLARE statement associates a cursor with a
 * PREPAREd statement.  The cursor name, like the statement
 * name, does not appear in the Declare Section.

 * A single cursor name cannot be DECLAREd more than once.
 */
    EXEC SQL DECLARE C CURSOR FOR S;

/* The OPEN statement evaluates the active set of the
 * PREPAREd query USING the specified input host variables,
 * which are substituted positionally for placeholders in
 * the PREPAREd query.  For each occurrence of a
 * placeholder in the statement there must be a variable
 * in the USING clause.  That is, if a placeholder occurs
 * multiple times in the statement, the corresponding
 * variable must appear multiple times in the USING clause.

 * The USING clause can be omitted only if the statement
 * contains no placeholders.  OPEN places the cursor at the
 * first row of the active set in preparation for a FETCH.

 * A single DECLAREd cursor can be OPENed more than once,
 * optionally USING different input host variables.
 */
    EXEC SQL OPEN C USING :deptno;

/* Break the loop when all data have been retrieved. */

    EXEC SQL WHENEVER NOT FOUND DO break;

/* Loop until the NOT FOUND condition is detected. */

    for (;;)
    {
/* The FETCH statement places the select list of the
 * current row into the variables specified by the INTO
 * clause, then advances the cursor to the next row.  If
 * there are more select-list fields than output host
 * variables, the extra fields will not be returned.
 * Specifying more output host variables than select-list
 * fields results in an ORACLE error.
 */
        EXEC SQL FETCH C INTO :ename;

/* Null-terminate the array before output. */
        ename.arr[ename.len] = '\0';
        puts((char *) ename.arr);
        }

/* Print the cumulative number of rows processed by the
 * current SQL statement.
 */
    printf("\nQuery returned %d row%s.\n\n", sqlca.sqlerrd[2],
        (sqlca.sqlerrd[2] == 1) ? "" : "s");

/* The CLOSE statement releases resources associated with
 * the cursor.
 */
    EXEC SQL CLOSE C;

/* Commit any pending changes and disconnect from Oracle. */
    EXEC SQL COMMIT RELEASE;
    puts("Sayonara.\n");
    exit(0);
}

void
dyn_error(msg)
char *msg;
{
    printf("\n%s", msg);
    sqlca.sqlerrm.sqlerrmc[sqlca.sqlerrm.sqlerrml] = '\0';
    oraca.orastxt.orastxtc[oraca.orastxt.orastxtl] = '\0';
    oraca.orasfnm.orasfnmc[oraca.orasfnm.orasfnml] = '\0';
    printf("\n%s\n", sqlca.sqlerrm.sqlerrmc);
    printf("in \"%s...\"\n", oraca.orastxt.orastxtc);
    printf("on line %d of %s.\n\n", oraca.oraslnr,
       oraca.orasfnm.orasfnmc);

/* Disable ORACLE error checking to avoid an infinite loop
 * should another error occur within this routine.
 */
    EXEC SQL WHENEVER SQLERROR CONTINUE;

/* Release resources associated with the cursor. */
    EXEC SQL CLOSE C;

/* Roll back any pending changes and disconnect from Oracle. */  
    EXEC SQL ROLLBACK RELEASE;
    exit(1);
}

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 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.

The 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

The 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

  See Also:

  Chapter 15, "Oracle Dynamic SQL: Method 4" for information on the SQLDA structure and variable names.

Implementing Oracle Method 4

With Oracle 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; 

However, select and bind descriptors need not work in tandem. So, 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 Method 3 OPEN statement

EXEC SQL OPEN cursor_name [USING host_variable_list]; 

with the Method 4 FETCH statement.

EXECUTE can be used for nonqueries with Method 4.

Restriction

In Dynamic SQL Method 4, you cannot bind a host array to a PL/SQL procedure with a parameter of type "table."

Using Host Arrays

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

EXEC SQL EXECUTE statement_name USING host_array_list; 

where host_array_list contains one or more host arrays.

Similarly, to use input 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.

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, just use Method 2 instead.

With Oracle 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.