Return column name and distinct values

Question

Say I have a simple table in postgres as the following:

+--------+--------+----------+
|  Car   |  Pet   |   Name   |
+--------+--------+----------+
| BMW    |           


        

Answer 1

It's bad style to put set-returning functions in the SELECT list and not allowed in the SQL standard. Postgres supports it for historical reasons, but since LATERAL was introduced Postgres 9.3, it's largely obsolete. We can use it here as well:

SELECT x.col, x.val
FROM   tbl, LATERAL (VALUES ('car', car)
                          , ('pet', pet)
                          , ('name', name)) x(col, val)
GROUP  BY 1, 2
ORDER  BY 1, 2;

You'll find this LATERAL (VALUES ...) technique discussed under the very same question on dba.SE you already found yourself. Just don't stop reading at the first answer.

Up until Postgres 9.4 there was still an exception: "parallel unnest" required to combine multiple set-returning functions in the SELECT list. Postgres 9.4 brought a new variant of unnest() to remove that necessity, too. More:

• Unnest multiple arrays in parallel

The new function is also does not derail into a Cartesian product if the number of returned rows should not be exactly the same for all set-returning functions in the SELECT list, which is (was) a very odd behavior. The new syntax variant should be preferred over the now outdated one:

SELECT DISTINCT x.*
FROM   tbl t, unnest('{car, pet, name}'::text[]
                   , ARRAY[t.car, t.pet, t.name]) AS x(col, val)
ORDER  BY 1, 2;

Also shorter and faster than two unnest() calls in parallel.

Returns:

 col  |  val
------+--------
 car  | BMW
 car  | Honda
 car  | Toyota
 name | Mary
 name | Sam
 pet  | Cat
 pet  | Dog

DISTINCT or GROUP BY, either is good for the task.