CODEWITHSUNDEEP
mysqlforeign-keysconstraints
Jasir
Jasir

Reputation: 697

What is the use of CONSTRAINT keyword when adding a Foreign key?

I Can't figure out what is the difference between these two.

CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id) REFERENCES ..

and

FOREIGN KEY (P_Id) REFERENCES ..

Is it just naming, or something else?

Upvotes: 8

Views: 3327

Answers (2)

Matteo Baldi
Matteo Baldi

Reputation: 5828

With the CONSTRAINT clause you can define a constraint name for the foreign key constraint. If missing MySQL will generate a name automatically.

Upvotes: 5

Shadow
Shadow

Reputation: 34232

As MySQL manual on foreign keys indicates, the CONSTRAINT symbol_name part of the constraint syntax is optional:

[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (index_col_name, ...)
REFERENCES tbl_name (index_col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION

The difference is in the naming of the foreign key. As the above linked document describes:

Otherwise, MySQL implicitly creates a foreign key index that is named according to the following rules:

• If defined, the CONSTRAINT symbol value is used. Otherwise, the FOREIGN KEY index_name value is used.

• If neither a CONSTRAINT symbol or FOREIGN KEY index_name is defined, the foreign key index name is generated using the name of the referencing foreign key column.

Upvotes: 6

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