We can also create FOREIGN KEY constraint on a column when the table is already created, for that we use ALTER command. We can also define FOREIGN KEY constraint on multiple column, for this use the below statement: CREATE TABLE user_addressĬONSTRAINT fk_user_address FOREIGN KEY (user_id) REFERENCES user(id) The following statement creates a FOREIGN KEY constraint on the 'user_id' column when the 'user_address' table is created:įOREIGN KEY (user_id) REFERENCES user(id) We cannot insert invalid data into the foreign key column as the FOREIN KEY constraint prevents that because the data must be exist in the primary key column on the another table it points to. In the above tables 'user_id' in the 'user_address' table points to the 'id' column in the 'user' table, where the 'id' column in the 'user' table is the PRIMARY KEY in the 'user' table and the 'user_id' column in the 'user_address' table is a FOREIGN KEY in the 'user_address' table. We have two tables with the following values.
In MySQL, the FOREIGN KEY on one table is used to point a PRIMARY KEY in another table. Note that the 'P_Id' column in the 'Orders' table points to the 'P_Id' column in the 'Persons' table.
