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SQL: The Complete Reference
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For the SQL2 descriptor, these statements would again be converted to a pair of SET DESCRIPTOR statements:
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SET DESCRIPTOR parmdesc VALUE(:j + l) INDICATOR = -1; SET DESCRIPTOR parmdesc VALUE (:j + 1) INDICATOR = 0;
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Note again the use of the loop control variable to specify which item in the descriptor is being set, and the direct passing of data (in this case, constants) rather than the use of pointers to buffers in the SQLDA structure. Finally, the program in Figure 18-8 passes the actual parameter value typed by the user to the DBMS, via the SQLDA. The statements at callout 8 accomplish this for data of different types, by first converting the typed characters into binary representations of the data and placing the binary data into the data buffers pointed to by the SQLDA. Again, the conversion to SQL2 involves replacing these pointers and direct SQLDA manipulation with a SET DESCRIPTOR statement. For example, these statements pass the data and its length for a variable-length character string:
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length = strlen(inbuf); SET DESCRIPTOR parmdesc VALUE (:j + 1) DATA = :inbuf; SET DESCRIPTOR parmdesc VALUE (:j + 1) LENGTH = :length;
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For data items that do not require a length specification, passing the data is even easier, since only the DATA form of the SET DESCRIPTOR statement is required. It s also useful to note that SQL2 specifies implicit data type conversions between host variables (such as inbuf) and SQL data types. Following the SQL standard, it would be necessary for the program in Figure 18-8 to perform all of the data type conversion in the sscanf() functions. Instead, the data could be passed to the DBMS as character data, for automatic conversion and error detection. With the SQLDA finally set up as required, the program in Figure 18-8 executes the dynamic UPDATE statement with the passed parameters at callout 9, using an EXECUTE statement that specifies a SQLDA. The conversion of this statement to a SQL2 descriptor is straightforward; it becomes:
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EXECUTE updatestmt USING SQL DESCRIPTOR parmdesc;
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The keywords in the EXECUTE statement change slightly, and the name of the descriptor is specified instead of the name of the SQLDA. Finally, the program in Figure 18-8 should be modified like this to tell the DBMS to deallocate the SQL2 descriptor. The statement that does this is:
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DEALLOCATE DESCRIPTOR parmdesc;
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Dynamic SQL *
In a simple program like this one, the DEALLOCATE is not very necessary, but in a more complex real-world program with multiple descriptors, it s a very good idea to deallocate the descriptors when the program no longer requires them.
SQL2 and Dynamic SQL Queries
In the dynamic SQL statements of the preceding sections, the SQL2 descriptor, like the SQLDA it replaces, is used to pass parameter information from the host program to the DBMS, for use in dynamic statement execution. The SQL2 standard also uses the SQL descriptor in dynamic query statements where, like the SQLDA it replaces, it controls the passing of query result from the DBMS back to the host program. Figure 18-9 lists a DB2-style dynamic SQL query program. It s useful to examine how the program in Figure 18-9 would change to conform to the SQL2 standard. Again, the flow of the program remains identical under SQL2, but the specifics change quite a lot. The SQL2 forms of the dynamic SQL query-processing statements are shown in Figure 18-20.
PROGRAMMING WITH SQL
Figure 18-20.
SQL2 dynamic query-processing statements
SQL: The Complete Reference
The declaration of the cursor for the dynamic query, in callout 1 of Figure 18-9, remains unchanged under SQL2. The construction of the dynamic SELECT statement in callout 2 is also unchanged, as is the PREPARE statement of callout 3. The changes to the program begin at callout 4, where the program uses the DESCRIBE statement to obtain a description of the query results, which is returned in a SQLDA named qry_da. For SQL2, this DESCRIBE statement must be modified to refer to a SQL2 descriptor, which must have been previously allocated. Assuming the descriptor is named qrydesc, the statements would be:
ALLOCATE DESCRIPTOR qrydesc WITH MAX :colcount; DESCRIBE querystmt USING SQL DESCRIPTOR qrydesc;
The SQL2 form of the DESCRIBE statement has a parallel effect on the one it replaces. Descriptions of the query result columns are returned, column by column, into the SQL2 descriptor, instead of into the SQLDA. Because the descriptor is a DBMS structure, rather than an actual data structure in the program, the host program must retrieve the information from the descriptor, piece by piece, as required. The GET DESCRIPTOR statement performs this function, just as the SET DESCRIPTOR function performs the opposite function of putting information into the SQL2 descriptor. In the program of Figure 18-9, the statements at callout 5, which obtains the length of a particular column of query results from a SQLDA, would be replaced with this statement:
GET DESCRIPTOR qrydesc VALUE (:i + 1) :length = LENGTH; qry_var -> sqldat = malloc(length);
The statement at callout 5 that allocates buffers for each item of query results is still needed, but the method for telling the DBMS where to put the results changes for SQL2. Instead of placing the address of the program destination for each item into the SQLDA, the program must place these addresses into the SQL2 descriptor, using the SET DESCRIPTOR statement. The buffers for the indicator variables are not needed with the SQL2 descriptor. Instead, the information about whether a column contains a NULL value can be obtained from the descriptor for each row as it is fetched, as seen later in the program example. In this particular example, the number of columns in the query results are calculated by the program as it builds the query. The program could also obtain the number of columns from the SQL2 descriptor with this form of the GET DESCRIPTOR statement:
GET DESCRIPTOR qrydesc :colcount = COUNT;
Having obtained the description of the query results, the program performs the query by opening the cursor at callout 6. The simple form of the OPEN statement,
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Dynamic SQL *
without any input parameters, conforms to the SQL2 standard. If the dynamic query specified parameters, they could be passed to the DBMS either as a series of host variables or via a SQL2 descriptor. The SQL2 OPEN statement using host variables is identical to the DB2 style, shown in the program in Figure 18-13. The SQL2 OPEN statement using a descriptor is parallel to the SQL2 EXECUTE statement using a descriptor, and differs from the DB2 style. For example, the OPEN statement of Figure 18-14:
OPEN qrycursor USING DESCRIPTOR :parmda;
is changed for SQL2 into this OPEN statement:
OPEN qrycursor USING SQL DESCRIPTOR parmdesc;
The technique for passing input parameters to the OPEN statement via the SQL2 descriptor is exactly the same as that described earlier for the EXECUTE statement. Like the Oracle implementation of dynamic SQL, the SQL2 standard provides a way for the host program to obtain a description of the parameters in a dynamic query as well as a description of the query results. For the program fragment in Figure 18-14, this DESCRIBE statement:
DESCRIBE INPUT querystmt USING SQL DESCRIPTOR parmdesc;
will return, in the SQL2 descriptor named parmdesc, a description of each of the parameters that appears in the dynamic query. The number of parameters can be obtained with the GET DESCRIPTOR statement, retrieving the COUNT item from the descriptor. As with the Oracle implementation, the SQL2 standard can have two descriptors associated with a dynamic query. The input descriptor, obtained with the DESCRIBE INPUT statement, contains descriptions of the parameters. The output descriptor contains descriptions of the query results columns. The standard allows you to explicitly ask for the output description:
DESCRIBE OUTPUT querystmt USING SQL DESCRIPTOR qrydesc;
but the DESCRIBE OUTPUT form of the statement is the default, and the most common practice is to omit the keyword OUTPUT. Returning to the dynamic query example of Figure 18-9, the cursor has been opened at callout 7, and it s time to fetch rows of query results at callout 8. Again, the SQL2 form of the FETCH statement is slightly modified to use the SQL2-style descriptor:
FETCH sqlcurs USING SQL DESCRIPTOR qrydesc;
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