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MySQL Syntax to create Foreign Key?
The syntax to create a foreign key is as follows −
alter table yourSecondTableName ADD CONSTRAINT yourConstraintname FOREIGN KEY(yourForeignKeyColumnName) references yourFirstTableName (yourPrimaryKeyColumnName);
To understand the above syntax, let us create two tables. The query to create the first table is as follows −
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)
The query to create the second table is as follows −
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)
The above Department_Id int, not null references department(departmentID) does not create a foreign key. Now follow the above syntax to create a foreign key.
The query is as follows −
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:
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