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In the next example, I create a foreign key that is a table constraint. Unlike the previous example, I include the name of the referenced column in this constraint definition, even though it isn t necessary:
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CREATE TABLE CD_TITLES ( CD_TITLE_ID INT, CD_TITLE VARCHAR(60), PUBLISHER_ID INT, CONSTRAINT FK_PUBLISHER_ID FOREIGN KEY (PUBLISHER_ID) REFERENCES CD_PUBLISHERS (PUBLISHER_ID) );
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The last two lines of code are the constraint definition. The name of the constraint, FK_PUBLISHER_ID, follows the CONSTRAINT keyword. Constraint names aren t necessary because the DBMS will assign a system-generated name if one is not supplied. However, it s a good practice to supply your own because constraint names often appear in error messages when SQL statements attempt to violate constraints, and names you supply will be easier to recognize than ones the DBMS supplied for you. Following the constraint name, the FOREIGN KEY keywords indicate the type of constraint, which is followed by the referencing column name, PUBLISHER_ID. This is the name of the column on which the constraint is being placed. If there were multiple column names, they would be separated by commas. The name of the referencing column is then followed by the REFERENCES keyword, which is followed by the name of the referenced table, CD_PUBLISHERS. The name of the referenced column follows the name of the referenced table. That s all there is to it. Once the constraint is defined, you would not be able to place values in the PUBLISHER_ID column of the CD_TITLES table unless those values already existed in the primary key of the CD_PUBLISHERS table. You should note, however, that the values in the foreign key do not have to be unique, as they must be in the CD_PUBLISHERS primary key. Values in the foreign key can be repeated any number of times, unless the column is limited by a unique constraint. Before I move on to discussing the other elements of the FOREIGN KEY syntax, let s take a quick look at a foreign key that includes multiple columns. In Figure 4-6, there are two tables: PERFORMING_ARTISTS and ARTISTS_MUSIC_TYPES. The primary key on the PERFORMING_ARTISTS table is defined on the ARTIST_ NAME and ARTIST_DOB columns. The following SQL statement creates the ARTISTS_ MUSIC_TYPES table, which includes a foreign key made up of the ARTIST_NAME and DOB columns:
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CREATE TABLE ARTISTS_MUSIC_TYPES ( ARTIST_NAME VARCHAR(60), DOB DATE, TYPE_ID INT, CONSTRAINT FK_CD_ARTISTS FOREIGN KEY ( ARTIST_NAME, DOB ) REFERENCES PERFORMING_ARTISTS (ARTIST_NAME, ARTIST_DOB) );
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PERFORMING_ARTISTS ARTIST_NAME: VARCHAR(60) Jennifer Warnes Joni Mitchell Bing Crosby Patsy Cline Placido Domingo ARTIST_DOB: PLACE_OF_BIRTH: DATE VARCHAR(60) 1947-03-03 1943-11-07 Unknown Fort MacLeod, Alberta, Canada PSTR_IN_STK: BOOLEAN False Unknown True True False Unknown
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ARTISTS_MUSIC_TYPES ARTIST_NAME: VARCHAR(60) Jennifer Warnes Jennifer Warnes Joni Mitchell Joni Mitchell Joni Mitchell Bing Crosby Bing Crosby Patsy Cline Patsy Cline Placido Domingo DOB: DATE TYPE_ID: INT
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1947-03-03 10 1947-03-03 06 1943-11-07 10 1943-11-07 05 1943-11-07 12 1904-05-02 05 1904-05-02 13 1932-09-08 02 1932-09-08 10 1941-01-21 19
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1904-05-02 Tacoma, Washington, U.S.A. 1932-09-08 Winchester, VIrginia, U.S.A. 1941-01-21 Madrid, Spain Unknown
Luciano Pavarotti 1935-10-12
Luciano Pavarotti 1935-10-12 19
Figure 4-6
A foreign key made up of multiple columns
In this statement, there are two referencing columns (ARTIST_NAME and DOB) and two referenced columns (ARTIST_NAME, ARTIST_DOB). The ARTIST_NAME columns in the two tables have the same data type, and the DOB column has the same data type as the ARTIST_DOB column. As you can see, one of the referencing columns (DOB) has a different name than its referenced counterpart (ARTIST_DOB).
Ask the Expert
Q: A:
In Figure 4-6 and in the preceding examples, you created a FOREIGN KEY constraint on the ARTIST_NAME and DOB columns in the ARTISTS_MUSIC_ TYPES table. What would the primary key be for this table Remember that a primary key must uniquely identify each row in a table. However, because pairs of values in the ARTIST_NAME and DOB columns can be repeated (which means that they can be repeated in the individual columns as well), those two columns cannot be used by themselves as a primary key for this table. On the other hand, the TYPE_ID column can have repeating values as well, so that column cannot be used by itself.
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