與MySQL鮮明SUM數

Question

我有一個查詢：

SELECT 

    t1.name as Name 

    count (distinct t2.key) as Total 
    SUM (IF(t2.time = '12:00' , 1 , 0)) as QttMidDay 

FROM t1 

LEFT JOIN t2 on t1.key = t2.key 

GROUP BY t1.key 

的問題是，我該怎麼做就2º參數總和QttMidDay「條件計數」？

Answer 1

我猜你可以通過在之前聚合來解決你的問題。我最好的猜測是：

SELECT t1.name as Name, t2.Total, t2.QttMidDay 
FROM t1 LEFT JOIN 
    (SELECT COUNT(DISTINCT t2.key) as Total, 
      SUM(t2.time = '12:00') as QttMidDay 
     FROM t2 
     GROUP BY t2.key 
    ) t2 
    ON t1.key = t2.key; 

我不知道，如果COUNT(DISTINCT)是必要的，子查詢。