barcode generator vb.net download Triggers and the SQL Standard in Software

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Triggers and the SQL Standard
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Triggers were one of the most widely praised and publicized features of Sybase SQL Server when it was first introduced, and they have since found their way into many commercial SQL products. Although the SQL2 standard provided an opportunity to standardize the DBMS implementation of triggers, the standards committee included check constraints instead, leaving triggers for a subsequent version (SQL:1999, also known as SQL3). As the trigger and check-constraint examples in the preceding sections show, check constraints can be effectively used to limit the data that can be added to a table or modified in a table. However, unlike triggers, they lack the ability to cause an independent action in the database, such as adding a row or changing a data item in another table. Some experts have argued that triggers are a pollution of the data management function of a database, and that the functions performed by triggers belong in fourth generation languages (4GLs) and other database tools, rather than in the DBMS itself. While the debate continues, DBMS products have experimented with new trigger capabilities that extend beyond the database itself. These extended trigger capabilities allow modifications to data in 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 how extensively they should be used. However, there is no doubt that triggers have become a more and more important part of SQL in enterprise applications over the last several years.
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Data Integrity
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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: Required columns can be specified when a table is created, and the DBMS will prevent NULL values in these columns. Data validation is limited to data type checking in standard SQL, but many DBMS products offer other data validation features. Entity integrity constraints ensure that the primary key uniquely identifies each entity represented in the database. Referential integrity constraints ensure that relationships among entities in the database are preserved during database changes (inserts, updates, and deletes). The SQL 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. Business rules can be enforced by the DBMS through the trigger mechanism popularized by Sybase and SQL Server and later included in the SQL:1999 standard. 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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PART III
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Transaction Processing
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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:
Add the new order to the ORDERS table. Update the sales total for the salesperson who took the order. Update the sales total for the salesperson s office. 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 compromised. 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.
Part III:
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