qr barcode generator vb.net The SQL Standard and Dynamic SQL Queries in Software

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The SQL Standard and Dynamic SQL Queries
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In the dynamic SQL statements of the preceding sections, the SQL 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 SQL 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 SQL standard. Again, the flow of the program remains identical under standard SQL, but the specifics change quite a lot. The standard SQL forms of the dynamic SQL query-processing statements are shown in Figure 18-20. The declaration of the cursor for the dynamic query, in callout 1 of Figure 18-9, remains unchanged under the SQL standard. 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 standard SQL, this DESCRIBE statement must be modified to refer to a SQL descriptor, which must have been previously allocated. Assuming the descriptor is named qrydesc, the statements would be
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ALLOCATE DESCRIPTOR qrydesc WITH MAX :colcount; DESCRIBE querystmt USING SQL DESCRIPTOR qrydesc;
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The standard SQL 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 SQL 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
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Programming with SQL
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DESCRIBE OUTPUT INPUT DECLARE cursor-name
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statement-name USING SQL DESCRIPTOR descriptor-name
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CURSOR FOR statement-name INSENSITIVE SCROLL CURSOR FOR statement-name INSENSITIVE SCROLL
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ALLOCATE cursor-name
OPEN cursor-name USING host-variable , USING SQL DESCRIPTOR descriptor-name FETCH cursor-name INTO host-variable , INTO SQL DESCRIPTOR descriptor-name CLOSE cursor-name
FIGURE 18-20
Dynamic query-processing statements
information into the SQL 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 standard SQL. Instead of placing the address of the program destination for each item into the SQLDA, the program must place these addresses into the SQL descriptor, using the SET DESCRIPTOR statement. The buffers for the indicator variables are not needed with the SQL 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 SQL descriptor with this form of the GET DESCRIPTOR statement:
GET DESCRIPTOR qrydesc :colcount = COUNT;
18:
Dynamic SQL*
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, without any input parameters, conforms to the SQL standard. If the dynamic query specified parameters, they could be passed to the DBMS either as a series of host variables or via a SQL descriptor. The standard SQL OPEN statement using host variables is identical to the DB2 style, shown in the program in Figure 18-13. The standard SQL OPEN statement using a descriptor is parallel to the standard SQL 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 standard SQL into this OPEN statement:
OPEN qrycursor USING SQL DESCRIPTOR parmdesc;
The technique for passing input parameters to the OPEN statement via the SQL descriptor is exactly the same as that described earlier for the EXECUTE statement. Like the Oracle implementation of dynamic SQL, the SQL 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 SQL descriptor named parmdesc, a description of each of the parameters that appear 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 SQL 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 6, and it s time to fetch rows of query results at callout 7. Again, the standard SQL form of the FETCH statement is slightly modified to use the standard SQL descriptor:
FETCH sqlcurs USING SQL DESCRIPTOR qrydesc;
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