How to convert column names to multiple rows? - sql

Question

I've a legacy database containing a table with multiple columns of type boolean. E.g.:
Table_1
id name has_lights has_engine has_brakes has_tyres can_move
1 bullock_cart false false false true true
2 car true true true true true
3 tank true true true false true
I'd like to write an SQL query for Table1 to fetch the id and name and the attributes (represented by the name of the column) that are true.
Expected output:
id name attributes
-- ---- ----------
1 bullock_cart has_tyres
1 bullock_cart can_move
2 car has_lights
2 car has_engine
2 car has_brakes
2 car has_tyres
2 car can_move
3 tank has_lights
3 tank has_engine
3 tank has_brakes
3 tank can_move
I wrote:
SELECT id, name,
CASE
WHEN has_lights THEN 'has_lights'
WHEN has_engine THEN 'has_engine'
WHEN has_brakes THEN 'has_brakes'
WHEN has_tyres THEN 'has_tyres'
WHEN can_move THEN 'can_move'
END
FROM TABLE1;
But this gets me only the 1st matching attribute for each row in Table1 (by virtue of CASE-WHEN).
What is the correct way to retrieve the data in the format I want? Any inputs/help will be greatly appreciated?
Notes:
The table structure isn't ideal but this is a legacy system and we cannot modify the schema.
Nested queries are ok as long as they aren't too slow - say for the sample above (I understand the number of matching rows/column factor in the slow-ness).

Answer 1

The simplest method is union all:
select id, name, 'has_lights' as attribute from t where has_lights union all
select id, name, 'has_engine' from t where has_engine union all
select id, name, 'has_brakes' from t where has_brakes union all
select id, name, 'has_tyres' from t where has_tyres union all
select id, name, 'can_move' from t where can_move;
If you have a very large table, then a lateral join is probably more efficient:
select t.id, t.name, v.attribute
from t, lateral
(select attribute
from (values (has_lights, 'has_lights'),
(has_engine, 'has_engine'),
(has_brakes, 'has_brakes'),
(has_tyres, 'has_tyres'),
(can_move, 'can_move')
) v(flag, attribute)
where flag
) v;

Answer 2

You can do it using UNION ALL :
SELECT name,'has_lights' as attributes FROM YourTable where has_lights = 'TRUE'
UNION ALL
SELECT name,'has_engine' as attributes FROM YourTable where has_engine= 'TRUE'
UNION ALL
SELECT name,'has_brakes' as attributes FROM YourTable where has_brakes = 'TRUE'
UNION ALL
SELECT name,'has_tyres' as attributes FROM YourTable where has_tyres = 'TRUE'
UNION ALL
SELECT name,'can_move' as attributes FROM YourTable where can_move = 'TRUE'

Answer 3

This is much like the brilliant query #Gordon posted:
SELECT t.id, t.name, v.attribute
FROM table1 t
JOIN LATERAL (
VALUES (has_lights, 'has_lights')
, (has_engine, 'has_engine')
, (has_brakes, 'has_brakes')
, (has_tyres , 'has_tyres')
, (can_move , 'can_move')
) v(flag, attribute) ON v.flag;
Just a bit shorter since the VALUES expression can stand on its own.