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The FETCH statement advances the cursor to the next row of query results and brings the values for that row into the program buffers, as specified within the descriptor structure. The program must still use the descriptor to determine information about each column of returned results, such as its length or whether it contains a NULL value. For example, to determine the returned length of a column of character data, the program might use the statement:
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GET DESCRIPTOR qrydesc VALUE(:i + 1) :length = RETURNED_LENGTH;
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Programming with SQL
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To determine whether the value in the column was NULL, the program can use the statement:
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And similarly to determine the data type of the column, the program can use the statement:
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As you can see, the details of row-by-row query processing within the for loop of the program will differ dramatically from those in Figure 18-9. Having processed all rows of query results, the program closes the cursor at callout 8. The CLOSE statement remains unchanged under standard SQL. Following the closing of the cursor, it would be good practice to deallocate the SQL descriptor(s), which would have been allocated at the very beginning of the program. The changes required to the dynamic SQL programs in Figures 18-8, 18-9, and 18-14 to make them conform to the SQL standard illustrate, in detail, the new features specified by the standard and the degree to which they differ from common dynamic SQL usage today. In summary, the changes from DB2-style dynamic SQL are The SQLDA structure is replaced with a named SQL descriptor. The ALLOCATE DESCRIPTOR and DEALLOCATE DESCRIPTOR statements are used to create and destroy descriptors, replacing allocation and deallocation of host program SQLDA data structures. Instead of directly manipulating elements of the SQLDA, the program specifies parameter values and information through the SET DESCRIPTOR statement. Instead of directly manipulating elements of the SQLDA, the program obtains information about query results and obtains the query result data itself through the GET DESCRIPTOR statement. The DESCRIBE statement is used both to obtain descriptions of query results (DESCRIBE OUTPUT) and to obtain descriptions of parameters (DESCRIBE INPUT). The EXECUTE, OPEN, and FETCH statements are slightly modified to specify the SQL descriptor by name instead of the SQLDA.
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Summary
This chapter described dynamic SQL, an advanced form of embedded SQL. Dynamic SQL is rarely needed to write simple data processing applications, but it is crucial for building general-purpose database front-ends. Static SQL and dynamic SQL present a classic tradeoff between efficiency and flexibility, which can be summarized as follows: Simplicity Static SQL is relatively simple; even its most complex feature, cursors, can be easily understood in terms of familiar file input/output concepts. Dynamic SQL is complex, requiring dynamic statement generation, variable-length data structures, and memory management, with memory allocation/deallocation, data type alignment, pointer management, and associated issues.
18:
Dynamic SQL*
Performance Static SQL is compiled into an application plan at compile-time; dynamic SQL must be compiled at runtime. As a result, static SQL performance is generally much better than that of dynamic SQL. The performance of dynamic SQL is dramatically impacted by the quality of the application design; a design that minimizes the amount of compilation overhead can approach static SQL performance. Flexibility Dynamic SQL allows a program to decide at runtime which specific SQL statements it will execute. Static SQL requires that all SQL statements be coded in advance, when the program is written, limiting the flexibility of the program. Dynamic SQL uses a set of extended embedded SQL statements to support its dynamic features: The EXECUTE IMMEDIATE statement passes the text of a dynamic SQL statement to the DBMS, which executes it immediately. The PREPARE statement passes the text of a dynamic SQL statement to the DBMS, which compiles it into an application plan but does not execute it. The dynamic statement may include parameter markers whose values are specified when the statement is executed. The EXECUTE statement asks the DBMS to execute a dynamic statement previously compiled by a PREPARE statement. It also supplies parameter values for the statement that is to be executed. The DESCRIBE statement returns a description of a previously prepared dynamic statement into a SQLDA. If the dynamic statement is a query, the description includes a description of each column of query results. The DECLARE CURSOR statement for a dynamic query specifies the query by the statement name assigned to it when it was compiled by the PREPARE statement. The OPEN statement for a dynamic query passes parameter values for the dynamic SELECT statement and requests query execution. The FETCH statement for a dynamic query fetches a row of query results into program data areas specified by a SQLDA structure. The CLOSE statement for a dynamic query ends access to the query results.
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