PostgreSQL 9.3:Обединяване на две колони в една

Добре оптимизиран:

WITH t(a, b, ct) AS (
   SELECT columna, columnb, count(*)::int
   FROM   test_1
   GROUP  BY 1, 2
   )
, matrix AS (
   SELECT a.*, b.*, COALESCE(t.ct, 0) AS ct
   FROM        (SELECT DISTINCT ON (a) a, right(a, -1)::int AS sorta FROM t) a
   CROSS  JOIN (SELECT DISTINCT ON (b) b, right(b, -1)::int AS sortb FROM t) b
   LEFT   JOIN t USING (a, b)
   )
(   
SELECT a AS ab, array_agg('"' || b || '" = ' || ct ORDER BY sortb) AS x
FROM   matrix
GROUP  BY 1, sorta
ORDER  BY sorta
)
UNION ALL
(
SELECT b      , array_agg('"' || a || '" = ' || ct ORDER BY sorta)
FROM   matrix
GROUP  BY 1, sortb
ORDER  BY sortb
);

SQL Fiddle.

Всички скоби са задължителни.

• Сумирайте резултатите от няколко заявки и след това намерете топ 5 в SQL