與MySQL計數不同的SUM

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

問題是，我如何在QttMidDay的2º參數SUM上進行“有條件計數”？

Answer 1

我猜想您可以通過在加入之前進行匯總來解決您的問題。 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;

我不確定子查詢中是否需要COUNT(DISTINCT) 。