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The DECLARE CURSOR Statement
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The DECLARE CURSOR statement, shown in Figure 17-24, defines a query to be performed. The statement also associates a cursor name with the query. The cursor name must be a valid SQL identifier. It is used to identify the query and its results in other embedded SQL statements. The cursor name is specifically not a host language variable; it is declared by the DECLARE CURSOR statement, not in a host language declaration. The SELECT statement in the DECLARE CURSOR statement defines the query associated with the cursor. The SELECT statement can be any valid interactive SQL SELECT statement, as described in s 6 through 9. In particular, the SELECT statement must include a FROM clause and may optionally include WHERE, GROUP BY, HAVING, and ORDER BY clauses. The SELECT statement may also include the UNION operator, as described in 6. Thus, an embedded SQL query can use any of the query capabilities that are available in the interactive SQL language.
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Figure 17-24.
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The DECLARE CURSOR statement syntax diagram
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The query specified in the DECLARE CURSOR statement may also include input host variables. These host variables perform exactly the same function as in the embedded INSERT, DELETE, UPDATE, and singleton SELECT statements. An input host variable can appear within the query anywhere that a constant can appear. Note that output host variables cannot appear in the query. Unlike the singleton SELECT statement, the SELECT statement within the DECLARE CURSOR statement has no INTO clause and does not retrieve any data. The INTO clause appears as part of the FETCH statement, described shortly. As its name implies, the DECLARE CURSOR statement is a declaration of the cursor. In most SQL implementations, including the IBM SQL products, this statement is a directive for the SQL precompiler; it is not an executable statement, and the precompiler does not produce any code for it. Like all declarations, the DECLARE CURSOR statement must physically appear in the program before any statements that reference the cursor that it declares. Most SQL implementations treat the cursor name as a global name that can be referenced inside any procedures, functions, or subroutines that appear after the DECLARE CURSOR statement. It s worth noting that not all SQL implementations treat the DECLARE CURSOR statement strictly as a declarative statement, and this can lead to subtle problems. Some SQL precompilers actually generate code for the DECLARE CURSOR statement (either host language declarations or calls to the DBMS, or both), giving it some of the qualities of an executable statement. For these precompilers, the DECLARE CURSOR statement must not only physically precede the OPEN, FETCH, and CLOSE statements that reference its cursor, but it must sometimes precede these statements in the flow of execution or be placed in the same block as the other statements. In general, you can avoid problems with the DECLARE CURSOR statement by following these guidelines: I Place the DECLARE CURSOR statement right before the OPEN statement for the cursor. This placement ensures the correct physical statement sequence, it puts the DECLARE CURSOR and the OPEN statements in the same block, and it ensures that the flow of control passes through the DECLARE CURSOR statement, if necessary. It also helps to document just what query is being requested by the OPEN statement. I Make sure that the FETCH and CLOSE statements for the cursor follow the OPEN statement physically as well as in the flow of control.
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The OPEN Statement
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The OPEN statement, shown in Figure 17-25, conceptually opens the table of query results for access by the application program. In practice, the OPEN statement actually causes the DBMS to process the query, or at least to begin processing it. The OPEN statement thus causes the DBMS to perform the same work as an interactive SELECT statement, stopping just short of the point where it produces the first row of query results.
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