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In addition, you probably wouldn t want to combine the TYPE_ID column with one of the other two columns because it is conceivable that you would have repeating rows (for example, two artists with the same name performing the same types of music, such as the two blues musicians named Sonny Boy Williamson, or two artists with the same date of birth performing the same type of music). As a result, your best solution (aside from adding another column to the table) is to roll all three columns into the primary key. Together, the three columns would uniquely identify each row because it is highly unlikely that anyone would share the same name, date of birth, and type of music (although anything is possible, which is why, ultimately, adding another column that is guaranteed to be unique is the very best way to go).
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The MATCH Clause
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Now that you have an understanding of how to define a basic FOREIGN KEY constraint, let s look at another line of the FOREIGN KEY syntax: [ MATCH { FULL | PARTIAL | SIMPLE } ] You can tell from the brackets that this is an optional clause. And in fact, very few vendor products currently support this clause (it s not supported by SQL Server 2005, Oracle 11g, or MySQL 5.0, for example), so you won t see it used much at all. However, it is described in the SQL Standard, which means we can expect more vendor product support in the future. Its purpose is to allow you to decide how to treat null values in the foreign key columns, with regard to permitting values to be inserted into the referencing columns. If the columns do not permit null values, then the MATCH clause does not apply. You have three options that you can use in the MATCH clause:
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If MATCH FULL is specified, all referencing columns must have a null value or none of these columns can have a null value. If MATCH PARTIAL is specified, one or more referencing columns can have null values as long as the remaining referencing columns have values that equal their corresponding referenced columns. If MATCH SIMPLE is specified and one or more referencing columns have null values, then the remaining referencing columns can have values that are not contained in the corresponding referenced columns. The SIMPLE option is implied if the MATCH clause is not included in the FOREIGN KEY constraint definition.
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The best way to illustrate each of these MATCH options is through examples of valid and invalid data that can be inserted in the referencing columns. Going back to our example shown in Figure 4-6, you can see that the foreign key in the ARTISTS_MUSIC_TYPES table is made up of two referencing columns: ARTIST_NAME and DOB. Table 4-1 provides examples for data that can and cannot be inserted into the foreign key columns. The examples are based on data in the primary key columns of the PERFORMING_ARTISTS table.
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MATCH Option
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Valid Data Examples
Joni Mitchell, 1943-11-07 NULL, NULL Patsy Cline, 1932-09-08 NULL, 1932-09-08 Patsy Cline, NULL NULL, NULL Bing Crosby, 1904-05-02 NULL, 1904-05-02 Bing Crosby, NULL NULL, 1802-08-03 Henryk G recki, NULL NULL, NULL
Invalid Data Examples
NULL, 1943-11-07 Joni Mitchell, NULL Joni Mitchell, 1802-08-03 NULL, 1802-08-03 Henryk G recki, NULL Patsy Cline, 1947-03-03 Bing Crosby, 1802-08-03 Bing Crosby, 1947-03-03 Henryk G recki, 1947-03-03
PARTIAL
SIMPLE
Table 4-1 Valid and Invalid Examples of the MATCH Clause Options
NOTE
You probably wouldn t want to permit null values in your referencing columns in the ARTISTS_MUSIC_TYPES table, particularly for the ARTIST_NAME column. And if either of these columns were used in the primary key, you would not be able to permit null values. However, in order to demonstrate how the MATCH options work, let s assume that null values are permitted.
If you decide to use the MATCH clause, you simply add it to the end of your FOREIGN KEY constraint definition, as shown in the following SQL statement (assuming your implementation of SQL supports it):
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 MATCH FULL );
To insert data into the referencing columns (ARTIST_NAME and DOB), both values have to be null or they must be valid data values from the referenced columns in the PERFORMING_ ARTISTS table.
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