ADD Foreign Key in existing Table

How to add Foreign Key in existing table

if you want to add any column as a foreign key in your existing table then you have to write following MySQL query for that

Syntax:

ALTER TABLE tablename ADD CONSTRAINT FOREIGN KEY column_name REFERENCES ref_table_name(column_name)

if you want to add multiple column as a foreign key in the same table then you have to write the following MySQL query:

Syntax:

ALTER TABLE tablename 
ADD CONSTRAINT FOREIGN KEY column_name REFERENCES ref_table_name(column_name)
ADD CONSTRAINT FOREIGN KEY column_name REFERENCES ref_table_name(column_name)
ADD CONSTRAINT FOREIGN KEY column_name REFERENCES ref_table_name(column_name)