Primary foreign key

Primary key defined

If the public key is the primary key in a relationship, then the public key is called a foreign key relationship to another

A table to the foreign key relationship to a primary key table is called the primary table, having a foreign key is called the primary table from the table

effect

Foreign keys for maintaining data consistency, integrity

The main purpose of the control data stored in the external key table

Design Principles

One pair of real user does not make sense
2 single unique
3 Do not update the primary key
4 primary key should not arbitrarily change the
5 primary key uniquely generated by the system should be based on policy

Foreign keys defined role

Foreign key (FK) for establishing one or more columns or strengthen the link between the two data tables.

By adding one or more columns in the primary key table to another table, to create a connection between the two tables, the foreign key column becomes the second table

FK constraints purpose is to control the data stored in the look, while controlling modifications to data primary key
the foreign key is used primarily to control the integrity of the data in the database, a data table when the operation is performed, and his associated data table or a plurality of tables can be changed simultaneously

外 键 onUpdate onDelete

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cascade primary key corresponding to the operation key corresponding to the outer operation followed industry. Such as removing updates

no action to ignore the effect of changes brought about by the primary key

restrict

SET null primary key value corresponding to the null where