Question

Question:

Why am I getting errors when trying to alter a table with a foreign key constraint?

Details:

I have 1 table, HSTORY which I use as a base table for all other specific history tables (ie. USER_HISTORY, BROWSER_HISTORY, PICTURE_HISTORY…). I have also included the PICTURE and USER tables as they get called as well.

HISTORY table:

CREATE TABLE IF NOT EXISTS HISTORY
(
   ID        INT NOT NULL AUTO_INCREMENT,
   VIEWERID  INT NOT NULL ,
   VIEWDATE  TIMESTAMP NOT NULL DEFAULT NOW(),

   PRIMARY KEY (ID),
   FOREIGN KEY (VIEWERID) REFERENCES USER(ID)
)
engine=innodb;

USER table: (in case anyone is curious)

CREATE TABLE IF NOT EXISTS USER
(
   ID        INT NOT NULL AUTO_INCREMENT,

   PRIMARY KEY (ID)
)
engine=innodb;

PICTURE table: (in case anyone is curious)

CREATE TABLE IF NOT EXISTS PICTURE
(
   ID        INT NOT NULL AUTO_INCREMENT,

   PRIMARY KEY (ID)
)
engine=innodb;

PICTURE_HISTORY table:

CREATE TABLE IF NOT EXISTS PICTURE_HISTORY LIKE HISTORY;

ALTER TABLE PICTURE_HISTORY
ADD FOREIGN KEY (FOREIGNID) REFERENCES PICTURE(ID);

However, when I do this, I get:

Key column 'FOREIGNID' doesn't exist in table

I take this to mean that I have to first create FOREIGNID, but in many of the examples on SO, the above should work. Anyone know why this is occurring?

Answer 1

Thanks to Michael for pointing out my mistake. I can’t actually make a foreign key unless the column already exists. If instead I issue these two commands, the foreign key constraint is created:

ALTER TABLE PICTURE_HISTORY
ADD COLUMN FOREIGNID INT NOT NULL;

ALTER TABLE PICTURE_HISTORY
ADD FOREIGN KEY (FOREIGNID) REFERENCES PICTURE(ID);