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a database to automatically cause actions such as sending mail, alerting a user, or launching another program to perform a task. This makes triggers even more useful and will add to the debate over including them in future official SQL standards. Regardless of the official stance, triggers have become a more and more important part of the SQL language in enterprise applications over the last several years.
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The SQL language provides a number of features that help to protect the integrity of data stored in a relational database: I Required columns can be specified when a table is created, and the DBMS will prevent NULL values in these columns. I Data validation is limited to data type checking in standard SQL, but many DBMS products offer other data validation features. I Entity integrity constraints ensure that the primary key uniquely identifies each entity represented in the database.
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I Referential integrity constraints ensure that relationships among entities in the database are preserved during database updates. I The SQL2 standard and newer implementations provide extensive referential integrity support, including delete and update rules that tell the DBMS how to handle the deletion and modification of rows that are referenced by other rows. I Business rules can be enforced by the DBMS through the trigger mechanism popularized by Sybase and SQL Server. Triggers allow the DBMS to take complex actions in response to events such as attempted INSERT, DELETE, or UPDATE statements. Check constraints provide a more limited way to include business rules in the definition of a database and have the DBMS enforce them.
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SQL: The Complete Reference
atabase updates are usually triggered by real-world events, such as the receipt of a new order from a customer. In fact, receiving a new order would generate not just one, but this series of four updates to the sample database:
I Add the new order to the ORDERS table. I Update the sales total for the salesperson who took the order. I Update the sales total for the salesperson s office. I Update the quantity-on-hand total for the ordered product. To leave the database in a self-consistent state, all four updates must occur as a unit. If a system failure or another error creates a situation where some of the updates are processed and others are not, the integrity of the database will be lost. Similarly, if another user calculates totals or ratios partway through the sequence of updates, the calculations will be incorrect. The sequence of updates must thus be an all-or-nothing proposition in the database. SQL provides precisely this capability through its transaction-processing features, which are described in this chapter.
What Is a Transaction
A transaction is a sequence of one or more SQL statements that together form a logical unit of work. The SQL statements that form the transaction are typically closely related and perform interdependent actions. Each statement in the transaction performs some part of a task, but all of them are required to complete the task. Grouping the statements as a single transaction tells the DBMS that the entire statement sequence should be executed atomically all of the statements must be completed for the database to be in a consistent state. Here are some examples of typical transactions for the sample database, along with the SQL statement sequence that comprises each transaction: I Add-an-order. To accept a customer s order, the order-entry program should (a) query the PRODUCTS table to ensure that the product is in stock, (b) insert the order into the ORDERS table, (c) update the PRODUCTS table, subtracting the quantity ordered from the quantity-on-hand of the product, (d) update the SALESREPS table, adding the order amount to the total sales of the salesperson who took the order, and (e) update the OFFICES table, adding the order amount to the total sales of the office where the salesperson works. I Cancel-an-order. To cancel a customer s order, the program should (a) delete the order from the ORDERS table, (b) update the PRODUCTS table, adjusting the quantity-on-hand total for the product, (c) update the SALESREPS table, subtracting the order amount from the salesperson s total sales, and (d) update the OFFICES table, subtracting the order amount from the office s total sales.
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