How to set foreign key constraints in MySQL?

MySQL foreign key constraint (FOREIGN KEY) is used to establish a link between the data of two tables. It can be one column or multiple columns. A table can have one or more foreign keys.

Foreign keys correspond to referential integrity. The foreign key of a table can be null. If it is not null, the value of each foreign key must be equal to a certain value of the primary key in another table.

Foreign key is a field of the table, not the primary key of this table, but corresponds to the primary key of another table. After a foreign key is defined, deleting related rows in another table is not allowed.

The main function of foreign keys is to maintain the consistency and integrity of data. For example, the primary key of the department table tb_dept is id, and there is a key deptId associated with this id in the employee table tb_emp5. Primary table (parent table): For two tables with an associated relationship, the table with the primary key in the associated field is the primary table. Dependent table (subtable): For two tables with an associated relationship, the table where the foreign key in the associated field is located is the secondary table.

Select the fields to set MySQL foreign key constraints

When defining a foreign key, you need to comply with the following rules:

The parent table must have Exists in the database, or is the table currently being created. If it is the latter case, the parent table and the child table are the same table. Such a table is called a self-referential table, and this structure is called self-referential integrity.

The primary key must be defined for the parent table.

The primary key cannot contain null values, but null values ​​are allowed in foreign keys. That is, as long as every non-null value of the foreign key appears in the specified primary key, the contents of the foreign key are correct.

Specify the column name or a combination of column names after the table name of the parent table. This column or combination of columns must be the primary key or candidate key of the parent table.

The number of columns in the foreign key must be the same as the number of columns in the primary key of the parent table.

The data type of the column in the foreign key must be the same as the data type of the corresponding column in the parent table's primary key.

Set foreign key constraints when creating the table

Use the FOREIGN KEY keyword to create foreign keys in the data table. The specific syntax rules are as follows :

[CONSTRAINT <外键名>] FOREIGN KEY 字段名 [，字段名2，…]
REFERENCES <主表名> 主键列1 [，主键列2，…]

Among them: the foreign key name is the name of the defined foreign key constraint. There cannot be a foreign key with the same name in a table; the field name indicates the field column that needs to add the foreign key constraint to the sub-table; the name of the main table That is, the name of the table on which the foreign key of the subtable depends; the primary key column represents the primary key column or column combination defined in the main table.

Example: In order to show the foreign key relationship between tables, this example creates a department table tb_dept1 in the test_db database. The table structure is as shown in the following table.

The result of running the SQL statement to create tb_dept1 is as follows.

mysql> CREATE TABLE tb_dept1
    -> (
    -> id INT(11) PRIMARY KEY,
    -> name VARCHAR(22) NOT NULL,
    -> location VARCHAR(50)
    -> );
Query OK, 0 rows affected (0.37 sec)

Create the data table tb_emp6, and create a foreign key constraint on the table tb_emp6, so that its key deptId is associated with the primary key id of the table tb_dept1 as a foreign key. The input SQL statement and running results are as follows.

mysql> CREATE TABLE tb_emp6
    -> (
    -> id INT(11) PRIMARY KEY,
    -> name VARCHAR(25),
    -> deptId INT(11),
    -> salary FLOAT,
    -> CONSTRAINT fk_emp_dept1
    -> FOREIGN KEY(deptId) REFERENCES tb_dept1(id)
    -> );
Query OK, 0 rows affected (0.37 sec)
mysql> DESC tb_emp6;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int(11)     | NO   | PRI | NULL    |       |
| name   | varchar(25) | YES  |     | NULL    |       |
| deptId | int(11)     | YES  | MUL | NULL    |       |
| salary | float        | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
4 rows in set (1.33 sec)

After the above statement is successfully executed, a foreign key constraint named fk_emp_dept1 is added to table tb_emp6. The foreign key name is deptId, which depends on the primary key id of table tb_dept1.

Tip: Association refers to the connection between related tables in a relational database. It is represented by the same attribute or group of attributes. The foreign key of the child table must be associated with the primary key of the parent table, and the data types of the associated fields must match. If the types are different, an error "ERROR 1005 (HY000): Can't create table' will occur when creating the child table. database.tablename'(errno:150)".

Add foreign key constraints when modifying the table

The syntax rules for adding foreign key constraints when modifying the data table are:

ALTER TABLE <数据表名> ADD CONSTRAINT <索引名>
FOREIGN KEY(<列名>) REFERENCES <主表名> (<列名>);

Example: Modify the data table tb_emp2, set the field deptId as a foreign key, and associate it with the primary key id of the data table tb_dept1. The input SQL statement and running results are as follows.

mysql> ALTER TABLE tb_emp2
    -> ADD CONSTRAINT fk_tb_dept1
    -> FOREIGN KEY(deptId)
    -> REFERENCES tb_dept1(id);
Query OK, 0 rows affected (1.38 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> SHOW CREATE TABLE tb_emp2\G
*************************** 1. row ***************************
       Table: tb_emp2
Create Table: CREATE TABLE `tb_emp2` (
  `id` int(11) NOT NULL,
  `name` varchar(30) DEFAULT NULL,
  `deptId` int(11) DEFAULT NULL,
  `salary` float DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_tb_dept1` (`deptId`),
  CONSTRAINT `fk_tb_dept1` FOREIGN KEY (`deptId`) REFERENCES `tb_dept1` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312
1 row in set (0.12 sec)

Delete foreign key constraints

For foreign keys defined in the database, you can delete them if they are no longer needed. Once the foreign key is deleted, the association between the master table and the slave table will be released. The syntax format for deleting the foreign key in MySQL is as follows:

ALTER TABLE <表名> DROP FOREIGN KEY <外键约束名>;

Example: Delete the foreign key in the data table tb_emp2 The key constraint fk_tb_dept1, the input SQL statement and the running result are as follows.

mysql> ALTER TABLE tb_emp2
    -> DROP FOREIGN KEY fk_tb_dept1;
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> SHOW CREATE TABLE tb_emp2\G
*************************** 1. row ***************************
       Table: tb_emp2
Create Table: CREATE TABLE `tb_emp2` (
  `id` int(11) NOT NULL,
  `name` varchar(30) DEFAULT NULL,
  `deptId` int(11) DEFAULT NULL,
  `salary` float DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_tb_dept1` (`deptId`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312
1 row in set (0.00 sec)

It can be seen that FOREIGN KEY no longer exists in tb_emp2, and the original foreign key constraint named fk_emp_dept was deleted successfully.

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