MySQL MIN/MAX vrací správnou hodnotu, ale ne související informace o záznamu

Stali jste se obětí laxních pravidel MySQL, která umožňují zahrnout neagregáty do dotazu GROUP BY. Jistě, pracujete s MIN nebo MAX a pouze ONE najednou, ale zvažte tento dotaz:

SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as earliest,
    MAX(created_timestamp) as latest
  FROM conditions
  GROUP BY condition_id

Nyní se zamyslete nad tím, který řádek sloupec stavu by měl pocházet z. Je absurdní dávat korelaci mezi agregované (ty ve GROUP BY) a neagregované sloupce.

Místo toho napište svůj dotaz takto

SELECT X.condition_id, C.status, X.earliest
FROM (
  SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as earliest
  FROM conditions
  GROUP BY condition_id
) X JOIN conditions C
  on CONCAT(c.work_type, c.work_id) = X.condition_id
  and c.created_timestamp = X.earliest

Ale pokud jste měli dva záznamy se stejným created_timestamp, bude to ještě složitější

SELECT X.condition_id, Max(C.status) status, X.earliest
FROM (
  SELECT 
    CONCAT(work_type, work_id) AS condition_id, 
    status,
    MIN(created_timestamp) as earliest
  FROM conditions
  GROUP BY condition_id
) X JOIN conditions C
  on CONCAT(c.work_type, c.work_id) = X.condition_id
  and c.created_timestamp = X.earliest
GROUP BY X.condition_id, X.earliest