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15 mar 2011 · You cannot modify the key in a single statement, see the ALTER TABLE syntax, in which there is no ALTER CONSTRAINT available. You must use 2 ALTER TABLE statements to accomplish what you want. Delete the key in the first one using an ALTER TABLE DROP FOREIGN KEY.
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.
There isn't much information about how to alter foreign keys in MySQL. There is no command like: ALTER TABLE my_table CHANGE FOREIGN KEY ( key_name ) REFERENCES other_table ( other_key ) ON DELETE SET NULL ;
You can simply use one query to rule them all: ALTER TABLE products DROP FOREIGN KEY oldConstraintName, ADD FOREIGN KEY (product_id, category_id) REFERENCES externalTableName (foreign_key_name, another_one_makes_composite_key) ON DELETE CASCADE ON UPDATE CASCADE
Summary: in this tutorial, you will learn about the MySQL Foreign Key constraint and how to create, drop and disable a foreign key constraint. MySQL Foreign Key Introduction. A foreign key is a column (or set of columns) that establishes a relationship between data in two tables.
The essential syntax for a defining a foreign key constraint in a CREATE TABLE or ALTER TABLE statement includes the following: [CONSTRAINT [symbol]] FOREIGN KEY. [index_name] (col_name, ...) REFERENCES tbl_name (col_name,...) [ON DELETE reference_option] [ON UPDATE reference_option] reference_option:
Summary: in this tutorial, you will learn about MySQL foreign key and how to create, drop, and disable a foreign key constraint. Introduction to MySQL foreign key. A foreign key is a column or group of columns in a table that links to a column or group of columns in another table.
