MySQL Lesson three

MySQL FOREIGN key

1. What is a foreign key

A specific relationship between a table and a table. Maintain the integrity and consistency of your data.

User table

Order Orders Form

1.1 Delete the user, do not delete the order, the data is inconsistent

Insert record in 1.2 order table

Foreign Key control: Let the user do not update the data, or the user delete the data when the order synchronization also deleted

2. Foreign Key Features:

2.1 To see if a foreign key exists:

2.1.1 InnoDB type

Show CREATE TABLE name:

engine--Storage Engine

The 2.1.2 foreign key is a constraint relationship between two tables.

2.1.3 The name of the foreign key is unique.

3. Create key FOREIGN key

CREATE TABLE table name (column definition index definition foreign key definition [constraint constraint name] foreign key[foreign key field]

references[foreign Key Name] (Foreign key field)

[on Delete{retrict|cascade|set null | no action}]

[on Update{restrict|cascade|set null|no Action}])

Restrict: Deny delete or update to parent table

Cascade: Delete or update of the parent table, automatically delete or update the corresponding record in the child table.

Set NULL parent table Delete, UPDATE, setting Word table foreign key field null

No action: Do not act.

To delete a foreign key:

MySQL Lesson three