MySQL在LEFT JOIN表达式内更改列

Question

I'm trying to use different columns based on their values. If column A is null then use column B. The thing is, this is inside a LEFT JOIN expression:

SELECT *
FROM 
  novinarji
  LEFT JOIN naslovi ON novinarji.stalniNaslovId=naslovi.id
  LEFT JOIN posta ON naslovi.postaId=posta.id
WHERE
  posta.stPoste = 3000

I need novinarji.stalniNaslovId to change to novinarji.zacasniNaslovId when the first column is NULL .

I hope this is clear enough.

Thank you!

Answer 1

You do not necessarily need to use onyl literal column names in a joining ON condition necessarily. The join's ON condition needs to result in a truthful boolean expression. You can therefore use COALESCE() in the join condition to return the preferred column or the alternate if the preferred is NULL .

-- Careful with SELECT * in a join query...
-- better to be specific about the needed column names
SELECT *
FROM 
  novinarji
  -- COALESCE() will return stalniNaslovId if non-null, and zacasniNaslovId otherwise
  LEFT JOIN naslovi 
    ON COALESCE(novinarji.stalniNaslovId, novinarji.zacasniNaslovId) = naslovi.id
  LEFT JOIN posta 
    ON naslovi.postaId=posta.id
    -- Move this condition to the ON clause, otherwise it turns
    -- your LEFT JOIN into an INNER JOIN if filtered in the WHERE
    AND post.stPoste = 3000