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The <referential triggered action> Clause
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The final clause in the FOREIGN KEY constraint syntax is the optional <referential triggered action> clause. The clause allows you to define what types of actions should be taken when attempting to update or delete data from the referenced columns if that attempt would cause a violation of the data in the referencing columns. For example, suppose you try to delete data from a table s primary key. If that primary key is referenced by a foreign key and if the data to
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be deleted is stored in the foreign key, then deleting the data from the primary key would cause a violation of the FOREIGN KEY constraint. Data in referencing columns must always be included in the referenced columns. The point to remember about the <referential triggered action> clause is that you are including in the definition of the referencing table (through the foreign key) an action that should be taken as a result of something being done to the referenced table. This can be clarified by taking a look at the syntax for the <referential triggered action> clause: ON UPDATE <referential action> [ ON DELETE <referential action> ] | ON DELETE <referential action> [ ON UPDATE <referential action> ] <referential action> ::= CASCADE | SET NULL | SET DEFAULT | RESTRICT | NO ACTION
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The ::= symbol (two consecutive colons plus an equals sign) is used in the SQL:2006 standard to separate a placeholder in the angle brackets from its definition. In the preceding syntax, the <referential action> placeholder is defined. The placeholder is used in the code preceding the definition. You would then take the definition (the five keywords) and use them in place of the <referential action> placeholder as it is used in the ON UPDATE and ON DELETE clauses.
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As you can see from the syntax, you can define an ON UPDATE clause, an ON DELETE clause, or both, and you can define them in any order. For each of these clauses you can choose one of five referential actions:
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If CASCADE is used and data is updated or deleted in the referenced columns, the data in the referencing columns is updated or deleted. If SET NULL is used and data is updated or deleted in the referenced columns, the values in the corresponding referencing columns are set to null. Null values have to be supported in the referencing columns for this option to work. If SET DEFAULT is used and data is updated or deleted in the referenced columns, the values in the corresponding referencing columns are set to their default values. Default values must be assigned to the referencing columns for this option to work. If RESTRICT is used and you try to update or delete data in your referenced columns that would cause a foreign key violation, you are prevented from performing that action. Data in the referencing columns can never violate the FOREIGN KEY constraint, not even temporarily. If NO ACTION is used and you try to update or delete data in your referenced columns that would cause a foreign key violation, you are prevented from performing that action. However, data violations can occur temporarily under certain conditions during the execution of an SQL statement, but the data in the foreign key is never violated in its final state (at the end of that execution). The NO ACTION option is the default used for both updates and deletes, if no referential triggered action is specified.
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If you decide to use the <referential triggered action> clause, you simply add it to the end of your FOREIGN KEY constraint definition, as shown in the following SQL statement:
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 ON UPDATE CASCADE ON DELETE CASCADE );
If you update data in or delete data from the referenced columns in PERFORMING_ ARTISTS, those changes will be made to the referencing columns in the ARTISTS_MUSIC_ TYPES table.
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