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The same was true for UNIQUE constraints. It wasn t until SQL-92 that null values were permitted in columns included in a UNIQUE constraint, which clearly set them apart from PRIMARY KEY constraints. Also, be wary of variations across vendor implementations. For example, Oracle will automatically add NOT NULL constraints to columns included in a PRIMARY KEY constraint, while SQL Server (or at least some versions of it) will display an error if you attempt to create a PRIMARY KEY constraint using columns that have not been specified with a NOT NULL constraint.
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Up to this point, the types of constraints that I ve discussed have had to do primarily with ensuring the integrity of data within a table. The NOT NULL constraint prevents the use of null values within a column, and the UNIQUE and PRIMARY KEY constraints ensure the uniqueness of values within a column or set of columns. However, the FOREIGN KEY constraint is different in that it is concerned with how data in one table relates to data in another table, which is why it is known as a referential constraint it references another table. (Actually, there is an exception called a recursive relationship where the foreign key refers to another row in the same table, but I m going to ignore this special case for now in order to focus on the basics.) You might recall from 1 that tables in a relational database are linked together in a meaningful way in order to ensure the integrity of the data. This association between tables forms a relationship that provides referential integrity between tables. Referential integrity prevents the manipulation of data in one table from adversely affecting data in another table. Let s take a look at an example that illustrates this point. Figure 4-5 shows two tables (CD_TITLES and CD_PUBLISHERS) that are each defined with a primary key.
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CD_TITLES CD_TITLE_ID: CD_TITLE: INT VARCHAR(60) 11001 11002 11003 11004 11005 11006 Famous Blue Raincoat Blue Past Light Kojiki That Christmas Feeling PUBLISHER_ID: INT 5422 5402 5412 5409 5403 CD_PUBLISHERS PUBLISHER_ID: COMPANY_NAME: INT VARCHAR(60) 5403 5402 5409 5412 5422 MCA Records Reprise Records Geffen Windham Hill Records Private Music
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The relationship between the CD_TITLES and CD_PUBLISHERS tables
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The CD_TITLE_ID column in the CD_TITLES table is configured with a PRIMARY KEY constraint, as is the PUBLISHER_ID column in the CD_PUBLISHERS table. Both these columns are shaded in the illustration. Notice that the CD_TITLES table contains a column named PUBLISHER_ID. This column includes values from the PUBLISHER_ID column of the CD_PUBLISHERS table. In fact, the PUBLISHER_ID values in the CD_TITLES table should include only values that come from the PUBLISHER_ID column in the CD_PUBLISHERS table. You should not be able to insert a row into CD_TITLES if the PUBLISHER_ID value is not listed in the CD_PUBLISHERS table. At the same time, if you alter or delete a PUBLISHER_ID value in the CD_PUBLISHERS table, you should be able to predict the outcome of your action if those same values exist in the CD_TITLES table. Under no circumstances would you want to delete a publisher and leave PUBLISHER_ID values in the CD_TITLES table that reference a publisher that no longer exists. These results can be achieved by using a FOREIGN KEY constraint. A FOREIGN KEY constraint enforces referential integrity between two tables by ensuring that no action is taken on either table that adversely affects the data protected by the constraint. In the tables shown in Figure 4-5, the FOREIGN KEY constraint must be configured on the PUBLISHER_ID column of the CD_TITLES table. The FOREIGN KEY constraint restricts the values in that column to the values of a candidate key (usually the primary key) in the related table. Only valid data values are permitted in the FOREIGN KEY column or columns.
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