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the database, turning on and off the deferral of constraint checking associated with the table contents.
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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: I No customer is allowed to place orders that would exceed the customer s credit limit. I The sales vice president must be notified whenever any customer is assigned a credit limit higher than $50,000. I Orders may remain on the books for only 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: I 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. I 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. Placing the burden of enforcing business rules on the application programs that access the database has several disadvantages: I 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. I Lack of consistency. If several programs written by different programmers handle updates to a table, they will probably enforce the rules somewhat differently.
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I Maintenance problems. If the business rules change, the programmers must identify every program that enforces the rules, then locate the code and modify it correctly. I 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. Triggers and the enforcement of business rules that they provide have been especially useful in enterprise database environments. When hundreds of application programs are being developed or modified every year by dozens of application programmers, the ability to centralize the definition and administration of business rules can be very useful.
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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: I The SALES column for the salesperson who took the order should be increased by the amount of the order. I 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:
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