Example: A SELECT statement using an allocated SQL descriptor

 

Suppose that your application needs to handle a dynamic SELECT statement that changes from one use to the next. This statement can be read from a display, passed from another application, or built dynamically by your application.

In other words, you don't know exactly what this statement is going to be returning every time. Your application needs to be able to handle the varying number of result columns with data types that are unknown ahead of time.

For example, the following statement needs to be processed:

   SELECT WORKDEPT, PHONENO 
     FROM CORPDATA.EMPLOYEE      WHERE LASTNAME = 'PARKER'

This SELECT statement has no INTO clause. Dynamic SELECT statements must not have an INTO clause, even if they return only one row.

The statement is assigned to a host variable. The host variable, in this case named DSTRING, is then processed by using the PREPARE statement as shown:

EXEC SQL PREPARE S1 FROM :DSTRING;

Next, determine the number of result columns and their data types. To do this, allocate the largest number of entries for an SQL descriptor that you think you will need. Assume that no more than 20 columns are ever expected to be accessed by a single SELECT statement.

EXEC SQL ALLOCATE DESCRIPTOR 'mydescr' WITH MAX 20;

Now that the descriptor is allocated, the DESCRIBE statement can be done to get the column information.

EXEC SQL DESCRIBE S1 USING DESCRIPTOR 'mydescr';

When the DESCRIBE statement is run, SQL places values that provide information about the statement's select-list into the SQL descriptor area defined by 'mydescr'. If the DESCRIBE determines that not enough entries were allocated in the descriptor, SQLCODE +239 is issued. As part of this diagnostic, the second replacement text value indicates the number of entries that are needed. The following code sample shows how this condition can be detected and shows the descriptor allocated with the larger size.

/* Determine the returned SQLCODE from the DESCRIBE statement */
EXEC SQL   GET DIAGNOSTICS CONDITION 1: returned_sqlcode = DB2_RETURNED_SQLCODE;

if returned_sqlcode = 239 then do;

/* Get the second token for the SQLCODE that indicated 
     not enough entries were allocated */

EXEC SQL 
    GET DIAGNOSTICS CONDITION 1: token = DB2_ORDINAL_TOKEN_2;             
  /* Move the token variable from a character host variable into an integer host variable */
  EXEC SQL 
    SET :var1 = :token;                             
  /* Deallocate the descriptor that is too small */
  EXEC SQL     DEALLOCATE DESCRIPTOR 'mydescr';                
  /* Allocate the new descriptor to be the size indicated by the retrieved token */
  EXEC SQL 
    ALLOCATE DESCRIPTOR 'mydescr' WITH MAX :var1;   
  /* Perform the describe with the larger descriptor */
  EXEC SQL 
   DESCRIBE s1 USING DESCRIPTOR 'mydescr';         
end;            

At this point, the descriptor contains the information about the SELECT statement. Now you are ready to retrieve the SELECT statement results. For dynamic SQL, the SELECT INTO statement is not allowed. You must use a cursor.

EXEC SQL   DECLARE C1 CURSOR FOR S1;

You will notice that the prepared statement name is used in the cursor declaration instead of the complete SELECT statement. Now you can loop through the selected rows, processing them as you read them. The following code sample shows how this is done.

EXEC SQL   OPEN C1;

EXEC SQL     FETCH C1 USING SQL DESCRIPTOR 'mydescr';
do while not at end of data;

 /* process current data returned  (see below for discussion of doing this) */

/* then read the next row */

 EXEC SQL     FETCH C1 USING SQL DESCRIPTOR 'mydescr';
end;

EXEC SQL   CLOSE C1;

The cursor is opened. The result rows from the SELECT statement are then returned one at a time using a FETCH statement. On the FETCH statement, there is no list of output host variables. Instead, the FETCH statement tells SQL to return results into the descriptor area. After the FETCH has been processed, you can use the GET DESCRIPTOR statement to read the values. First, read the header value that indicates how many descriptor entries were used.

EXEC SQL   GET DESCRIPTOR 'mydescr' :count = COUNT;

Next you can read information about each of the descriptor entries. After you determine the data type of the result column, you can do another GET DESCRIPTOR to return the actual value. To get the value of the indicator, specify the INDICATOR item. If the value of the INDICATOR item is negative, the value of the DATA item is not defined. Until another FETCH is done, the descriptor items will maintain their values.

do i = 1 to count;
  GET DESCRIPTOR 'mydescr' VALUE :i  /* set entry number to get */
                         :type = TYPE,                    /* get the data type */
                         :length = LENGTH,                /* length value */
                         :result_ind = INDICATOR;            
 if result_ind >= 0 then 
    if type = character 
     GET DESCRIPTOR 'mydescr' VALUE :i                         :char_result = DATA;       /* read data into character field */
    else     if type = integer        GET DESCRIPTOR 'mydescr' VALUE :i      
                        :int_result = DATA;       /* read data into integer field */
  else 
      /* continue checking and processing for all data types that might be returned */
end;

There are several other descriptor items that you might need to check to determine how to handle the result data. PRECISION, SCALE, DB2_CCSID, and DATETIME_INTERVAL_CODE are among them. The host variable that has the DATA value read into it must have the same data type and CCSID as the data being read. If the data type is varying length, the host variable can be declared longer than the actual data. For all other data types, the length must match exactly.

NAME, DB2_SYSTEM_COLUMN_NAME, and DB2_LABEL can be used to get name-related values for the result column. See GET DESCRIPTOR for more information about the items returned for a GET DESCRIPTOR statement and for the definition of the TYPE values

 

Parent topic:

Processing SELECT statements and using a descriptor