In Postgresql, force unique on combination of two columns

Question

I would like to set up a table in PostgreSQL such that two columns together must be unique. There can be multiple values of either value, so long as there are not two that s

Answer 1

If, like me, you landed here with:

• a pre-existing table,
• to which you need to add a new column, and
• also need to add a new unique constraint on the new column as well as an old one, AND
• be able to undo it all (i.e. write a down migration)

Here is what worked for me, utilizing one of the above answers and expanding it:

-- up

ALTER TABLE myoldtable ADD COLUMN newcolumn TEXT;
ALTER TABLE myoldtable ADD CONSTRAINT myoldtable_oldcolumn_newcolumn_key UNIQUE (oldcolumn, newcolumn);

---

ALTER TABLE myoldtable DROP CONSTRAINT myoldtable_oldcolumn_newcolumn_key;
ALTER TABLE myoldtable DROP COLUMN newcolumn;

-- down