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Foreign key constraints and modeling
For the relationship of mysql, we can use modeling software to describe the database table in detail. Both PowerDesigner and sqlyog can be used to build the relationship model.
The advantage of PowerDesigner is that it can make Chinese comments on tables, and sqlyog cannot have Chinese comments.
The schematic diagram of PowerDesigner modeling is as follows:
sqlyog modeling requires clicking on the new schema designer:
The schematic diagram of the modeling is as follows:
Foreign key constraints can double-click the relationship line to add foreign key constraints. There are four different options for foreign key constraints: CASCADE, SET NULL, NO ACTION, RESTRICT.
1.CASCADE: Delete or update the corresponding row from the parent table, and automatically delete or update the matching row from the table. [ON DELETE CANSCADE and ON UPDATE CANSCADE are both supported by InnoDB. ]
2.SET NULL: Delete or update the corresponding row from the parent table, and set the foreign key column in the child table to be empty. [Note that these only work if the foreign key column is not set to NOT NULL. Both ON DELETE SET NULL and ON UPDATE SET SET NULL are supported by InnoDB. ]
3.NO ACTION: InnoDB refuses to delete or update the parent table.
4. RESTRICT: Refuse to delete or update the parent table. [Specifying RESTRICT (or NO ACTION) has the same effect as ignoring the ON DELETE or ON UPDATE options. ]
The operation diagram is as follows:
Four options can be set in the drop-down list.