Query to determine if columns combine to create a unique key
If the table has a decent amount of data, it's probably easier to do
SELECT t.a, t.b, t.c, count(*) FROM my_table t GROUP BY t.a, t.b, t.cHAVING COUNT(*) > 1If that query returns 0 rows, the three columns are (currently) unique. If that query returns 1 or more rows, you'll know which values are duplicated.
Of course, if you find that the three columns are currently unique, you'll want to create a unique constraint if you intend to make use of that fact.