MySQL - Make a pair of values unique

Question

I have a table with two int values that are IDs. On their own these IDs can show up any number of times in the table, but together they should only ever appear once.

Answer 1

It's called a composite key.

If you want to change your actual PK to a composite one, use

Alter table  drop PRIMARY KEY;
Alter table  drop COLUMN ;

Alter table  add [constraint ] PRIMARY KEY (, );

You can also just add a unique constraint (your PK will be the same, and unique pairs... will have to be unique).

alter table  add [constraint ] unique index(, );

Personnally, I would recommend the second solution (simple PK + unique constraint), but that's just a personal point of view. You can google for pros and cons arguments about composite keys.

The part between [] are optional.

EDIT

If you wanna do this in the create table statement

For a composite pk

CREATE TABLE Test(
    id1 int NOT NULL, 
    id2 int NOT NULL,
    id3 int NOT NULL,
    PRIMARY KEY (id1, id2)
);

For an unique index

CREATE TABLE Test1(
    id1 int NOT NULL AUTO_INCREMENT, 
    id2 int NOT NULL,
    id3 int NOT NULL,
    PRIMARY KEY (id1),
    UNIQUE KEY (id2, id3)
);