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Many of the data integrity issues in the real world have to do with the rules and procedures of an organization. For example, the company that is modeled by the sample database might have rules like these: No customer is allowed to place orders that would exceed the customer's credit limit. The sales vice president must be notified whenever any customer is assigned a credit limit higher than $50,000. Orders may remain on the books only for six months; orders older than six months must be canceled and reentered. In addition, there are often "accounting rules" that must be followed to maintain the integrity of totals, counts, and other amounts stored in a database. For the sample database, these rules probably make sense: Whenever a new order is taken, the SALES column for the salesperson who took the order and for the office where that salesperson works should be increased by the order amount. Deleting an order or changing the order amount should also cause the SALES columns to be adjusted. Whenever a new order is taken, the QTY_ON_HAND column for the product being ordered should be decreased by the quantity of products ordered. Deleting an order, changing the quantity, or changing the product ordered should also cause corresponding adjustments to the QTY_ON_HAND column. These rules fall outside the realm of the SQL language as defined by the SQL1 standard and as implemented by many SQL-based DBMS products today. The DBMS takes responsibility for storing and organizing data and ensuring its basic integrity, but enforcing the business rules is the responsibility of the application programs that access the database.
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Placing the burden of enforcing business rules on the application programs that access the database has several disadvantages: Duplication of effort. If six different programs deal with various updates to the ORDERS table, each of them must include code that enforces the rules relating to ORDERS updates. Lack of consistency. If several programs written by different programmers handle updates to a table, they will probably enforce the rules somewhat differently. Maintenance problems. If the business rules change, the programmers must identify every program that enforces the rules, locate the code, and modify it correctly. Complexity. There are often many rules to remember. Even in the small sample database, a program that handles order changes must worry about enforcing credit limits, adjusting sales totals for salespeople and offices, and adjusting the quantities on hand. A program that handles simple updates can become complex very quickly. The requirement that application programs enforce business rules is not unique to SQL. Application programs have had that responsibility since the earliest days of COBOL programs and file systems. However, there has been a steady trend over the years to put more "understanding" of the data and more responsibility for its integrity into the database itself. In 1986 the Sybase DBMS introduced the concept of a trigger as a step toward including business rules in a relational database. The concept proved to be very popular, so support for triggers began to appear in many SQL DBMS products in the early 1990s, including those of the mainstream enterprise DBMS vendors.
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What Is a Trigger
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The concept of a trigger is relatively straightforward. For any event that causes a change in the contents of a table, a user can specify an associated action that the DBMS should carry out. The three events that can trigger an action are attempts to INSERT, DELETE, or UPDATE rows of the table. The action triggered by an event is specified by a sequence of SQL statements. To understand how a trigger works, let's examine a concrete example. When a new order is added to the ORDERS table, these two changes to the database should also take place: The SALES column for the salesperson who took the order should be increased by the amount of the order. The QTY_ON_HAND amount for the product being ordered should be decreased by the quantity ordered. This Transact-SQL statement defines a SQL Server trigger, named NEWORDER, that causes these database updates to happen automatically: CREATE ON FOR AS TRIGGER NEWORDER ORDERS INSERT UPDATE SALESREPS SET SALES = SALES + INSERTED.AMOUNT FROM SALESREPS, INSERTED WHERE SALESREPS.EMPL_NUM = INSERTED.REP UPDATE PRODUCTS SET QTY_ON_HAND = QTY_ON_HAND - INSERTED.QTY
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FROM PRODUCTS, INSERTED WHERE PRODUCTS.MFR_ID = INSERTED.MFR AND PRODUCTS.PRODUCT_ID = INSERTED.PRODUCT 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: 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 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" (preUPDATE) values and "new" (post-UPDATE) values. The 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
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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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