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Here is the Oracle PL/SQL version of the same loop:
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/* Lower targets until total below $2,400,000 */ select sum(target) into total_tgt from offices; while (total_tgt < 2400000.00) loop update offices set target = target 10000.00; select sum(target) into total_tgt from offices; end loop;
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The subquery-style version of the SELECT statement from Transact-SQL has been replaced by the PL/SQL SELECT...INTO form of the statement, with a local variable used to hold the total of the office targets. Each time the loop is executed, the OFFICES table is updated, and then the total of the targets is recalculated. Here is the same loop once more, expressed using Informix SPL s WHILE statement:
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/* Lower targets until total below $2,400,000 */ select sum(target) into total_tgt from offices; while (total_tgt < 2400000.00) update offices set target = target 10000.00; select sum(target) into total_tgt from offices; end while;
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Other variants of these loop-processing constructs are provided by the various dialects, but the capabilities and syntax are similar to these examples.
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Some stored procedure dialects provide statements to control looping and alter the flow of control. In Informix, for example, the EXIT statement interrupts the normal flow within a loop and causes execution to resume with the next statement following the loop itself. The CONTINUE statement interrupts the normal flow within the loop but causes execution to resume with the next loop iteration. Both of these statements have three forms, depending on the type of loop being interrupted:
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exit for; exit while; exit foreach; continue for; continue while; continue foreach;
In Transact-SQL, a single statement, BREAK, provides the equivalent of the Informix EXIT statement variants, and there is a single form of the CONTINUE statement as well. In Oracle, the EXIT statement performs the same function as for Informix, and there is no CONTINUE statement. Additional control over the flow of execution within a stored procedure is provided by statement labels and the GOTO statement. In most dialects, the statement label is an identifier, followed by a colon. The GOTO statement names the label to which control should be transferred.
20:
Database Processing and Stored Procedural SQL
There is typically a restriction that you cannot transfer control out of a loop or a conditional testing statement, and always a prohibition against transferring control into the middle of such a statement. As in structured programming languages, the use of GOTO statements is discouraged, because it makes stored procedure code harder to understand and debug.
Cursor-Based Repetition
One common need for repetition of statements within a stored procedure is when the procedure executes a query and needs to process the query results, row by row. All of the major dialects provide a structure for this type of processing. Conceptually, the structures parallel the DECLARE CURSOR, OPEN CURSOR, FETCH, and CLOSE CURSOR statements in embedded SQL or in the corresponding SQL API calls. However, instead of fetching the query results into the application program, in this case, they are being fetched into the stored procedure, which is executing within the DBMS itself. Instead of retrieving the query results into application program variables (host variables), the stored procedure retrieves them into local stored procedure variables. To illustrate this capability, assume that you want to populate two tables with data from the ORDERS table. One table, named BIGORDERS, should contain customer name and order size for any orders over $10,000. The other, SMALLORDERS, should contain the salesperson s name and order size for any orders under $1000. The best and most efficient way to do this would be to use two separate SQL INSERT statements with subqueries, but for purposes of illustration, consider this method instead: 1. Execute a query to retrieve the order amount, customer name, and salesperson name for each order. 2. For each row of query results, check the order amount to see whether it falls into the proper range for including in the BIGORDERS or SMALLORDERS tables. 3. Depending on the amount, INSERT the appropriate row into the BIGORDERS or SMALLORDERS table. 4. Repeat Steps 2 and 3 until all rows of query results are exhausted. 5. Commit the updates to the database. Figure 20-13 shows an Oracle stored procedure that carries out this method. The cursor that defines the query is defined early in the procedure and assigned the name O_CURSOR. The variable CURS_ROW is defined as an Oracle row type. It is a structured Oracle row variable with individual components (like a C-language structure). By declaring it as having the same row type as the cursor, the individual components of CURS_ROW have the same data types and names as the cursor s query results columns. The query described by the cursor is actually carried out by the cursor-based FOR loop. It basically tells the DBMS to carry out the query described by the cursor (equivalent to the OPEN statement in embedded SQL) before starting the loop processing. The DBMS then executes the FOR loop repeatedly, by fetching a row of query results at the top of the loop, placing the column values into the CURS_ROW variable, and then executing the statements in the loop body. When no more rows of query results are to be fetched, the cursor is closed, and processing continues after the loop.
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