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Unlike primary keys, foreign keys in a relational database are allowed to contain NULL values. In the sample database the foreign key REP_OFFICE, in the SALESREPS table, permits NULL values. In fact, this column does contain a NULL value in Tom Snyder's
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row, because Tom has not yet been assigned to an office. But the NULL value poses an interesting question about the referential integrity constraint created by the primary key/foreign key relationship. Does the NULL value match one of the primary key values or doesn't it The answer is "maybe" it depends on the "real" value of the missing or unknown data. Both DB2 and the ANSI/ISO SQL1 standard automatically assume that a foreign key that contains a NULL value satisfies the referential integrity constraint. In other words, they give the row "the benefit of the doubt" and allow it to be part of the child table, even though its foreign key value doesn't match any row in the parent table. Interestingly, the referential integrity constraint is assumed to be satisfied if any part of the foreign key has a NULL value. This can produce unexpected and unintuitive behavior for compound foreign keys, such as the one that links the ORDERS table to the PRODUCTS table. Suppose for a moment that the ORDERS table in the sample database permitted NULL values for the PRODUCT column, and that the PRODUCTS/ORDERS relationship had a SET NULL delete rule. (This is not the actual structure of the sample database, for the reasons illustrated by this example.) An order for a product with a manufacturer id (MFR) of ABC and a NULL product id (PRODUCT) can be successfully inserted into the ORDERS table because of the NULL value in the PRODUCT column. DB2 and the ANSI/ISO standard assume that the row meets the referential integrity constraint for ORDERS and PRODUCTS, even though no product in the PRODUCTS table has a manufacturer id of ABC. The SET NULL delete rule can produce a similar effect. Deleting a row from the PRODUCTS table will cause the foreign key value in all of its child rows in the ORDERS table to be set to NULL. Actually, only those columns of the foreign key that accept NULL values are set to NULL. If there were a single row in the PRODUCTS table for manufacturer DEF, deleting that row would cause its child rows in the ORDERS table to have their PRODUCT column set to NULL, but their MFR column would continue to have the value DEF. As a result, the rows would have a MFR value that did not match any row in the PRODUCTS table. To avoid creating this situation, you should be very careful with NULL values in compound foreign keys. An application that enters or updates data in the table that contains the foreign key should usually enforce an "all NULLs or no NULLs" rule on the columns of the foreign key. Foreign keys that are partially NULL and partially non-NULL can easily create problems. The SQL2 standard addresses this problem by giving the database administrator more control over the handling of NULL values in foreign keys for integrity constraints. The integrity constraint in the CREATE TABLE statement provides two options: The MATCH FULL option requires that foreign keys in a child table fully match a primary key in the parent table. With this option, no part of the foreign key can contain a NULL value, so the issue of NULL value handling in delete and update rules does not arise. The MATCH PARTIAL option allows NULL values in parts of a foreign key, so long as the non-NULL values match the corresponding parts of some primary key in the parent table. With this option, NULL value handling in delete and update rules proceeds as previously described.
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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. The SQL2 standard goes beyond these capabilities to include a much more general capability for specifying and enforcing data integrity
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constraints. The complete scheme includes four types of constraints: Column constraints are specified as part of a column definition when a table is created. Conceptually, they restrict the legal values that may appear in the column. Column constraints appear in the individual column definitions within the CREATE TABLE statement. Domains are 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, using the CREATE DOMAIN statement. Table constraints are 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 SQL2 standard allows them to be interspersed with the column definitions. Assertions are the most general type of SQL2 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 which crosses multiple tables within the database. Each of the four different types of constraints has its own conceptual purpose, and each appears in a different part of the SQL2 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 realworld 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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