MySQL COUNT() på tværs af flere kolonner

SELECT
  COUNT(DISTINCT val_1) AS val_1_count,
  COUNT(DISTINCT val_2) AS val_2_count,
  ...
FROM ...

vil give dig tællingerne for hvert felt.

SELECT val_1, count(*) as val_1_count
FROM ...
GROUP BY val_1

vil give dig tæller for en værdi. Du kan bruge UNION til at gentage dette for val_1 til val_n i en enkelt (kludgy) forespørgsel.

Hvis du vil have tællingerne over alle felter, skal du have

SELECT val,count(*) as valcount
FROM (
  SELECT val_1 AS val FROM ...
  UNION ALL
  SELECT val_2 AS val FROM ...
  ...
) AS baseview
GROUP BY val