MySQL Basics (4) - Constraints

Overview: Constraints are rules that act on fields in a table to limit the data stored in the table.

Purpose: To ensure the correctness, validity and integrity of the data in the database.

Classification:

constraint	describe	keywords
not-null constraint	Restrict the data of this field to not be null	NOT NULL
unique constraint	Ensure that all data in this field is unique and not repeated	UNIQUE
primary key constraint	The primary key is the unique identifier of a row of data, which requires non-null and unique	PRIMARY KEY
default constraint	When saving data, if the value of this field is not specified, the default value will be used	DEFAULT
Check constraints (after version 8.0.1)	Ensure that the field value satisfies a certain condition	CHECK
foreign key constraints	It is used to establish a connection between the data of the two graphs to ensure the consistency and integrity of the data	FOREIGN KEY

Constraints are applied to the fields in the table, and constraints can be added when creating/modifying tables. Multiple constraints can be added to a field.

foreign key constraints

The foreign key is used to connect the data of the two tables to ensure the consistency and integrity of the data.

Add foreign key:

CREATE TABLE 表名(
    字段名 字段类型,
    ...
    [CONSTRAINT] [外键名称] FOREIGN KEY(外键字段名) REFERENCES 主表(主表列名)
);
ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名) REFERENCES 主表(主表列名);
-- 例子
alter table emp add constraint fk_emp_dept_id foreign key(dept_id) references dept(id);

Remove foreign keys:

ALTER TABLE 表名 DROP FOREIGN KEY 外键名;

delete/update behavior

Behavior	illustrate
NO ACTION	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, delete/update is not allowed (consistent with RESTRICT)
RESTRICT	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, delete/update is not allowed (consistent with NO ACTION)
CASCADE	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, delete/update the foreign key record in the child table
SET NULL	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, set the foreign key value in the child table to null (the foreign key is required to allow null)
SET DEFAULT	When the parent table is changed, the child table sets the foreign key to a default value (Innodb does not support)

Change delete/update behavior:

ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段) REFERENCES 主表名(主表字段名) ON UPDATE 行为 ON DELETE 行为;