Postgresql query to transpose columns to rows

Question

I have a table like this one:

id_edifc classe_a classe_b classe_c
9001 0 0 1
9002 1 1 1
9003 0 1 2
9004 0 0 0
9005 1 1 0

and I would like to visualize it like this instead:

id_edifc classe
9001 classe_c
9002 classe_a
9002 classe_b
9002 classe_c
9003 classe_b
9003 classe_c
9003 classe_c
9004 NULL
9005 classe_a
9005 classe_b

Answer 1

Using LATERAL:

SELECT t.id_edifc, s.name
FROM tab t
,LATERAL ( VALUES (classe_a, 'classe_a')
                 ,(classe_b, 'classe_b')
                 ,(classe_c, 'classe_c'))s(val, name)
WHERE val = 1;

To handle all zeros in row and replicating values:

SELECT t.id_edifc, s2.name
FROM tab t
LEFT JOIN LATERAL (SELECT s.name
                  FROM (VALUES  (classe_a, 'classe_a')
                               ,(classe_b, 'classe_b')
                               ,(classe_c, 'classe_c'))s(val, name)
                   ,LATERAL generate_series(1,s.val)
                   WHERE val > 0
                 ) s2 ON TRUE
ORDER BY t.id_edifc;

db<>fiddle demo

Output:

+-----------+----------+
| id_edifc  |   name   |
+-----------+----------+
|     9001  | classe_c |
|     9002  | classe_a |
|     9002  | classe_b |
|     9002  | classe_c |
|     9003  | classe_b |
|     9003  | classe_c |
|     9003  | classe_c |
|     9004  | NULL     |
|     9005  | classe_a |
|     9005  | classe_b |
+-----------+----------+