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Primary key and foreign key constraints, uniqueness constraints, and restrictions on missing (NULL) values all provide data integrity checking for very specific structures and situations within a database. Starting with SQL2, the SQL standard goes beyond these capabilities to include a much more general capability for specifying and enforcing data integrity constraints. The complete scheme includes four types of constraints: Column constraints Specified as part of a column definition when a table is created, or added later when a table is altered. Conceptually, they restrict the legal values that may appear in the column. Column constraints appear in the individual column definitions within the CREATE TABLE and ALTER TABLE statements. Domains A specialized form of column constraints. They provide a limited capability to define new data types within a database. In effect, a domain is one of the predefined database data types plus some additional constraints, which are specified as part of the domain definition. Once a domain is defined and named, the domain name can be used in place of a data type to define new columns. The columns inherit the constraints of the domain. Domains are defined outside of the table and column definitions of the database by using the CREATE DOMAIN statement. As already mentioned, very few SQL implementations provide support for this statement. Table constraints Specified as part of the table definition when a table is created. Conceptually, they restrict the legal values that may appear in rows of the table. Table constraints are specified in the CREATE TABLE statement that defines a table. Usually, they appear as a group after the column definitions, but the SQL standard allows them to be interspersed with the column definitions. Assertions The most general type of SQL constraint. Like domains, they are specified outside of the table and column structure of the database. Conceptually, an assertion specifies a relationship among data values that crosses multiple tables within the database. Unfortunately, also like domains, very few current SQL implementations support assertions. Each of the four different types of constraints has its own conceptual purpose, and each appears in a different part of SQL statement syntax. However, the distinctions between them are somewhat arbitrary. Any column constraint that appears for an individual column definition can just as easily be specified as a table constraint. Similarly, any table constraint can be specified as an assertion. In practice, it s probably best to specify each database constraint where it seems to most naturally fit, given the real-world situation that the database is trying to model. Constraints that apply globally to the entire situation (business processes, interrelationships among customers and products, and so on) should appear as assertions. Constraints that apply to a specific type of entity (a customer or an order) should appear as table constraints or column constraints within the appropriate table that describes that type of entity. When the same constraint applies to many different columns in the database that all refer to the same type of entity, then a domain is appropriate.
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Examples of the first three types of constraints have previously appeared in earlier sections of this chapter. An assertion is specified using the SQL CREATE ASSERTION statement. Here is an assertion that might be useful in the demo database: Ensure that an of ce s target does not exceed the sum of the quotas for its salespeople.
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CREATE ASSERTION target_valid CHECK ((OFFICES.TARGET <= SUM(SALESREPS.QUOTA)) AND (SALESREPS.REP_OFFICE = OFFICES.OFFICE));
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Because it is an object in the database (like a table or a column), the assertion must be given a name (in this case, it s target_valid). The name is used in error messages produced by the DBMS when the assertion is violated. The assertion causing an error may be obvious in a small demo database, but in a large database that might contain dozens or hundreds of assertions, it s critical to know which of the assertions was violated. Here is another example of an assertion that might be useful in the sample database: Ensure that the total of the orders for any customer does not exceed their credit limit.
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CREATE ASSERTION credit_orders CHECK (CUSTOMERS.CREDIT_LIMIT <= SELECT SUM(ORDERS.AMOUNT) FROM ORDERS WHERE ORDERS.CUST = CUSTOMERS.CUST_NUM);
As these examples show, a SQL assertion is defined by a search condition, which is enclosed in parentheses and follows the keyword CHECK. Every time an attempt is made to modify the contents of the database through an INSERT or UPDATE or DELETE statement, the search condition is checked against the (proposed) modified database contents. If the search condition remains TRUE, the modification is allowed. If the search condition would become untrue, the DBMS does not carry out the proposed modification, and an error code is returned, indicating an assertion violation. In theory, assertions could cause a large amount of database processing overhead as they are checked for each statement that might modify the database. In practice, the DBMS will analyze the assertion and determine which tables and columns it involves. Only changes that involve those particular tables or columns will actually trigger the search condition. Nonetheless, assertions should be defined with great care to ensure that they impose a reasonable amount of overhead for the benefit they provide.
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