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The table that contains the foreign key is the referencing table. The table that is being referenced by the foreign key is the referenced table. Likewise, the column or columns that make up the foreign key in the referencing table are referred to as the referencing columns. The columns being referenced by the foreign key are the referenced columns.
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When creating a FOREIGN KEY constraint, you must follow several guidelines:
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The referenced columns must be defined with either a UNIQUE or PRIMARY KEY constraint on them. As you might guess, the primary key is most commonly used for the referenced columns. A FOREIGN KEY constraint can be created as a table constraint or column constraint. If you create the foreign key as a column constraint, you can include only one column. If you create the foreign key as a table constraint, you can include one or more columns. The foreign key in the referencing table must include the same number of columns that are being referenced, and the referencing columns must each be configured with the same data types as their referenced counterparts. However, the referencing columns do not have to have the same names as the referenced columns. If you don t specify the referenced columns when you define a FOREIGN KEY constraint, then the columns defined in the primary key of the referenced table are used as the referenced columns.
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These guidelines will become clearer as I explain how to implement a foreign key. First, let s take a look at the basic syntax used to create that constraint. If you want to add a FOREIGN KEY constraint as a column constraint, you must add the constraint to a column definition, as shown in the following syntax: <column name> { <data type> | <domain> } [ NOT NULL ] REFERENCES <referenced table> [ ( <referenced columns> ) ] [ MATCH { FULL | PARTIAL | SIMPLE } ] [ <referential triggered action> ] If you want to add a FOREIGN KEY constraint as a table constraint, you must add it as a table element in the table definition, as shown in the following syntax: [ CONSTRAINT <constraint name> ] FOREIGN KEY ( <referencing column > [ {, <referencing column> } . . . ] ) REFERENCES <referenced table> [ ( <referenced columns> ) ] [ MATCH { FULL | PARTIAL | SIMPLE } ] [ <referential triggered action> ] As you can see, a FOREIGN KEY constraint is a bit more complicated than the constraint syntax you ve looked at so far. However, creating a basic FOREIGN KEY constraint is a relatively straightforward process. Let s take a look at one first, and then we ll go on to the more complex language elements. In the following example, I use a CREATE TABLE statement to create the CD_TITLES table (shown in Figure 4-5) and define a column constraint:
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CREATE TABLE CD_TITLES ( CD_TITLE_ID INT, CD_TITLE VARCHAR(60), PUBLISHER_ID INT REFERENCES CD_PUBLISHERS );
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This statement defines a FOREIGN KEY constraint on the PUBLISHER_ID column. Notice that, in order to add a column constraint, all you have to do is add the REFERENCES keyword and the name of the referenced table. Also notice that the foreign key contains the same number of columns as the primary key in the referenced table, and the referenced and referencing columns are the same data type. Remember, if you re not referencing the primary key in the referenced table, you must also include the name of the column or columns for example, REFERENCES CD_PUBLISHERS (PUBLISHER_ID).
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Before you can create a foreign key on a table, the referenced table must already exist and a UNIQUE or PRIMARY KEY constraint must be defined for that table.
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