Do composite primary key attributes have to be unique?

Question

Let's say that we create the following table:

CREATE TABLE example (
    a integer,
    b integer,
    c integer,
    PRIMARY KEY (a, c)
);

Obviously the compination of a and c has to be unique. But do a and c have to be unique themselves?

No. Only the combination has to be unique to qualify as a key. — sticky bit, May 21 '18 at 17:02
You could test this veeeeery quickly. Did you try to insert a test and see what happened? — dfundako, May 21 '18 at 17:04
I'm voting to close this question as off-topic because the meagrest of effort would reveal the answer. — Strawberry, May 21 '18 at 18:39

Lukasz Szozda · Accepted Answer · 2018-05-21T17:12:18.383

3

No they don't have to be unique separately. Only pairs should be unique.

Example:

a, c
1, 3
2, 3
2, 1
2, 1  -- this will cause unique key violation

INSERT INTO example(a,b,c) VALUES (1,2,3),(2,2,3),(2,3,1);

edited May 21 '18 at 17:12

answered May 21 '18 at 17:02

Lukasz Szozda

1 Answers1