MySQL 创建外键的语法?
mysqlmysqli database
创建外键的语法如下 −
alter table yourSecondTableName ADD CONSTRAINT yourConstraintname FOREIGN KEY(yourForeignKeyColumnName) references yourFirstTableName (yourPrimaryKeyColumnName);
为了理解上述语法，让我们创建两个表。创建第一个表的查询如下 −
mysql> create table Department_Table -> ( -> Department_Id int not null auto_increment primary key, -> Department_Name varchar(30) -> ); Query OK, 0 rows affected (0.83 sec)
创建第二张表的查询如下 −
mysql> create table Employee_Table -> ( -> EmployeeID int not null auto_increment primary key, -> EmployeeName varchar(80), -> Job varchar(30), -> Department_Id int not null references department(departmentID) -> ); Query OK, 0 rows affected (1.12 sec)
上面的 Department_Id int，not null 引用部门(departmentID)不会创建外键。现在按照上述语法创建外键。
查询如下 −
mysql> alter table Employee_Table ADD CONSTRAINT fk_Department_Id FOREIGN KEY(Department_Id) -> references Department_Table(Department_Id); Query OK, 0 rows affected (2.82 sec) Records: 0 Duplicates: 0 Warnings:
