postgres - transpose columns to rows - postgresql

Question

I need to transpose columns to rows in postgres, below is the requirement.
Any help is appreciated.
Source table/data :
id class-1-male class-1-female class-2-male class-2-female class-3-male class-3-female
1 1 1 11 7 0 9
2 11 31 6 7 40 92
3 15 31 8 37 30 91
4 11 13 50 17 10 19
I want data in below format:
id class-type male female
1 class-1 1 1
2 class-1 11 31
3 class-1 15 31
4 class-1 11 13
1 class-2 11 7
2 class-2 6 7
3 class-2 8 37
4 class-2 50 17
1 class-3 0 9
2 class-3 40 92
3 class-3 30 91
4 class-3 10 19

Answer 1

If you wanted to do the opposite (which is really more common) you could use hte tablefunc contrib module. But this way, you can simply use UNION ALL and query each column separately:
SELECT id, 'class-1' AS class-type,
class-1-male AS male, class-1-female AS female
FROM your_table
UNION ALL
SELECT id, 'class-2' AS class-type,
class-2-male AS male, class-2-female AS female
FROM your_table
UNION ALL
SELECT id, 'class-3' AS class-type,
class-3-male AS male, class-3-female AS female
FROM your_table
If you have too many rows on this table, this may not be the best option, because it reads the entire dataset for each query, which means it read them all three times when it could read just once. Another option would be creating a function that returns the set:
CREATE OR REPLACE FUNCTION classes_transpose()
RETURNS TABLE(id int, class_type text, male int, female int)
LANGUAGE PLPGSQL
STABLE
AS $$
DECLARE
v RECORD;
BEGIN
FOR v IN SELECT * FROM your_table LOOP
id := v.id;
class_type := 'class-1';
male := v."class-1-male";
female := v."class-1-female";
RETURN NEXT;
class_type := 'class-2';
male := v."class-2-male";
female := v."class-2-female";
RETURN NEXT;
class_type := 'class-3';
male := v."class-3-male";
female := v."class-3-female";
RETURN NEXT;
END LOOP;
END;
$$;
Then:
SELECT * FROM classes_transpose();