CREATE TRIGGER NEWORDER ON ORDERS FOR INSERT in Software

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CREATE TRIGGER NEWORDER ON ORDERS FOR INSERT
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AS UPDATE SET FROM WHERE UPDATE SET FROM WHERE AND
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SALESREPS SALES = SALES + INSERTED.AMOUNT SALESREPS, INSERTED SALESREPS.EMPL_NUM = INSERTED.REP PRODUCTS QTY_ON_HAND = QTY_ON_HAND - INSERTED.QTY PRODUCTS, INSERTED PRODUCTS.MFR_ID = INSERTED.MFR PRODUCTS.PRODUCT_ID = INSERTED.PRODUCT
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The first part of the trigger definition tells SQL Server that the trigger is to be invoked whenever an INSERT statement is attempted on the ORDERS table. The remainder of the definition (after the keyword AS) defines the action of the trigger. In this case, the action is a sequence of two UPDATE statements, one for the SALESREPS table and one for the PRODUCTS table. The row being inserted is referred to using the pseudo-table name inserted within the UPDATE statements. As the example shows, SQL Server extends the SQL language substantially to support triggers. Other extensions not shown here include IF/THEN/ELSE tests, looping, procedure calls, and even PRINT statements that display user messages. The trigger capability, while popular in many DBMS products, is not a part of the ANSI/ISO SQL2 standard. As with other SQL features whose popularity has preceded standardization, this has led to a considerable divergence in trigger support across various DBMS brands. Some of the differences between brands are merely differences in syntax. Others reflect real differences in the underlying capability. DB2 s trigger support provides an instructive example of the differences. Here is the same trigger definition shown previously for SQL Server, this time using the DB2 syntax:
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CREATE TRIGGER AFTER REFERENCING FOR EACH BEGIN UPDATE SET WHERE UPDATE SET WHERE AND END NEWORDER INSERT ON ORDERS NEW AS NEW_ORD ROW MODE DB2SQL ATOMIC SALESREPS SALES = SALES + NEW_ORD.AMOUNT SALESREPS.EMPL_NUM = NEW_ORD.REP; PRODUCTS QTY_ON_HAND = QTY_ON_HAND NEW_ORD.QTY PRODUCTS.MFR_ID = NEW_ORD.MFR PRODUCTS.PRODUCT_ID = NEW_ORD.PRODUCT;
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The beginning of the trigger definition includes the same elements as the SQL Server definition, but rearranges them. It explicitly tells DB2 that the trigger is to be
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invoked after a new order is inserted into the database. DB2 also allows you to specify that the trigger is to be carried out before a triggering action is applied to the database contents. This doesn t make sense in this example, because the triggering event is an INSERT operation, but it does make sense for UPDATE or DELETE operations. The DB2 REFERENCING clause specifies a table alias (NEW_ORD) that will be used to refer to the row being inserted throughout the remainder of the trigger definition. It serves the same function as the INSERTED keyword in the SQL Server trigger. The statement references the new values in the inserted row because this is an INSERT operation trigger. For a DELETE operation trigger, the old values would be referenced. For an UPDATE operation trigger, DB2 gives you the ability to refer to both the old (pre-UPDATE) values and new (post-UPDATE) values. BEGIN ATOMIC and END serve as brackets around the sequence of SQL statements that define the triggered action. The two searched UPDATE statements in the body of the trigger definition are straightforward modifications of their SQL Server counterparts. They follow the standard SQL syntax for searched UPDATE statements, using the table alias specified by the REFERENCING clause to identify the particular row of the SALESREPS table and the PRODUCTS table to be updated. The row being inserted is referred to using the pseudo-table name inserted within the UPDATE statements. Here is another example of a trigger definition, this time using Informix Universal Server:
CREATE TRIGGER NEWORDER INSERT ON ORDERS AFTER (EXECUTE PROCEDURE NEW_ORDER)
This trigger again specifies an action that is to take place after a new order is inserted. In this case, the multiple SQL statements that form the triggered action can t be specified directly in the trigger definition. Instead, the triggered statements are placed into an Informix stored procedure, named NEW_ORDER, and the trigger causes the stored procedure to be executed. As this and the preceding examples show, although the core concepts of a trigger mechanism are very consistent across databases, the specifics vary a great deal. Triggers are certainly among the least portable aspects of SQL databases today.
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