How do I enforce set-like uniqueness between multiple columns? [duplicate]

问题

This question already has answers here:

sql unique constraint on a 2 columns combination (3 answers)

Closed 5 years ago.

I'm not sure I've phrased the question correctly, so I'll try a longer explanation. I have this kind of table:

CREATE TABLE x (a int, b int);

I want to consider the pair (a,b) to be identical to (b,a), and to disallow insertion of duplicates. If PostgreSQL had a set data type, I might declare the table like this:

CREATE TABLE x (
    ab set,
    UNIQUE (ab)
);

But it doesn't, so what's the best way of doing this?

回答1:

create unique index idx_unique_ab 
    on x (least(a,b), greatest(a,b));

回答2:

I'd just enforce that a pair can only be stored one way round then create a regular unique constraint.

CREATE TABLE x
  (
     a INT,
     b INT,
     CHECK (a < b),
     UNIQUE(a, b)
  ); 

来源：https://stackoverflow.com/questions/13430658/how-do-i-enforce-set-like-uniqueness-between-multiple-columns