Dyntex.net Support Centre: MySQL Reference Manual p.515

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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

Chapter 7: MySQL Table Types 515 7.5.4.2 Foreign Key Constraints Starting from version 3.23.43b InnoDB features foreign key constraints. InnoDB is the rst MySQL table type which allows you to dene foreign key constraints to guard the integrity of your data. The syntax of a foreign key constraint denition in InnoDB: FOREIGN KEY (index_col_name, ...) REFERENCES table_name (index_col_name, ...) [ON DELETE CASCADE | ON DELETE SET NULL] Both tables have to be InnoDB type and there must be an index where the foreign key and the referenced key are listed as the rst columns. InnoDB does not auto-create indexes on foreign keys or referenced keys: you have to create them explicitly. Corresponding columns in the foreign key and the referenced key must have similar internal data types inside InnoDB so that they can be compared without a type conversion. The size and the signedness of integer types has to be the same. The length of string types need not be the same. Starting from version 3.23.50 you can also associate the ON DELETE CASCADE or ON DELETE SET NULL clause with the foreign key constraint. If ON DELETE CASCADE is specied, and a row in the parent table is deleted, then InnoDB automatically deletes also all those rows in the child table whose foreign key values are equal to the referenced key value in the parent row. If ON DELETE SET NULL is specied, the child rows are automatically updated so that the columns in the foreign key are set to the SQL NULL value. Starting from version 3.23.50, InnoDB does not check foreign key constraints on those foreign key or referenced key values which contain a NULL column. Starting from version 3.23.50 the InnoDB parser allows you to use backquotes (`) around table and column names in the above denition but the InnoDB parser is not yet aware of possible variable lower_case_table_names you give in `my.cnf'. An example: CREATE TABLE parent(id INT NOT NULL, PRIMARY KEY (id)) TYPE=INNODB; CREATE TABLE child(id INT, parent_id INT, INDEX par_ind (parent_id), FOREIGN KEY (parent_id) REFERENCES parent(id) ON DELETE SET NULL ) TYPE=INNODB; If MySQL gives the error number 1005 from a CREATE TABLE statement, and the error message string refers to errno 150, then the table creation failed because a foreign key constraint was not correctly formed. Similarly, if an ALTER TABLE fails and it refers to errno 150, that means a foreign key denition would be incorrectly formed for the altered table. Starting from version 3.23.50 InnoDB allows you to add a new foreign key constraint to a table through ALTER TABLE yourtablename ADD CONSTRAINT FOREIGN KEY (...) REFERENCES anothertablename(...) Remember to create the required indexes rst, though. In InnoDB versions < 3.23.50 ALTER TABLE or CREATE INDEX should not be used in connec- tion with tables which have foreign key constraints or which are referenced in foreign key

MySQL Reference Manual

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