What is FOREIGN KEY

FOREIGN KEY

FOREIGN  is column in a table that is the primary key column in another table. a table  can have  multiple foreign column in a table . It can hold multiple null values.

Example: Below are the EMPLOYEE and EMPSALARY table.

EMPLOYEE table:

CREATE TABLE EMPLOYEE(

       ID   INT              NOT NULL,

       NAME VARCHAR (20)     NOT NULL,

       AGE  INT              NOT NULL,

       ADDRESS  CHAR (25) ,

       SALARY   DECIMAL (18, 2),      

      CONSTRAINT pk_EmployeeID PRIMARY KEY (ID)

);

EMPSALARY table:

CREATE TABLE EMPSALARY (

       ID          INT       NOT NULL,

       DATE        DATETIME,

       EMPLOYEE_ID INT references EMPLOYEE(ID),

       AMOUNT     double,

       CONSTRAINT pk_ EMPSALARYID PRIMARY KEY (ID)

);

in above  EMPSALARY table EMPLOYEE_ID is the foreign key that refrenece to the primary key in the EMPLOYEE table.

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