Question

I wish to have a foreign key in a table but I have no idea how to do it. I wish have a UserID Column in the “wall” table so that I can link back and get the userid’s details etc. How would i go about doing this?

Wall Table:
alt text http://img821.imageshack.us/img821/7701/screenshot20100808at010.png

Users Table:
alt text http://img375.imageshack.us/img375/7701/screenshot20100808at010.png

Answer 1

1) In order to have a foreign key column called userid in the WALL table, you need to create the column – skip to #2 if the column already exists:

ALTER TABLE WALL
  ADD COLUMN userid INT(25) NOT NULL

If you want to allow WALL records to exist without the userid being specified:

ALTER TABLE WALL
  ADD COLUMN userid INT(25) NULL

2) Now you add the foreign key constraint:

ALTER TABLE WALL
 ADD FOREIGN KEY (userid) REFERENCES USERS(userid)