MySQL连接三个查询而不增加记录

Question

我有三张桌子：

tblUnits [Unit_ID, Country]

tblEvents [Unit_ID, Month, Duration_Hrs]

tblMetrics [Unit_ID, Month, Service_Hrs]

并在其上构建了 3 个查询：

1.

SELECT Unit_ID
FROM tblUnits
WHERE City = 'NewYork'

有输出：

Unit_ID
1
2
3

SELECT
COUNT(IF(Event_Type in ('X', 'Y'), 1, NULL)) as Ev_Count,
SUM(CASE
        WHEN Event_Type in ('X', 'Y') THEN Duration_Hrs
    END) Duration
FROM tblEvents
WHERE
Design = 'GoodDesign'
AND Month = 'May'
AND Unit_ID in (1,2,3)

有输出：

Ev_Count | Duration
1        | 10

SELECT SUM(Service_Hrs) as SumSH
FROM tblMetrics
WHERE Month = 'May'
AND Unit_ID in (1,2,3)

有输出：

SumSH
100

现在我想结合这 3 个查询来获得如下输出：

Ev_Count | Duration | SumSH
1        | 10       | 100

我的带有两个左连接的查询将 tblEvents 和 tblMetrics 值相乘。

Answer 1

如果没有额外的行，您将无法加入查询，这将返回错误的聚合，并且由于您的 MySql 版本不支持CTEs，您必须使用运算符IN 使用两次第一个查询：

SELECT
  SUM(Event_Type IN ('X', 'Y')) Ev_Count,
  SUM(CASE WHEN Event_Type IN ('X', 'Y') THEN Duration_Hrs END) Duration,
  (
    SELECT SUM(Service_Hrs)
    FROM tblMetrics
    WHERE Month = 'May'
    AND Unit_ID IN (SELECT Unit_ID FROM tblUnits WHERE City = 'NewYork')
  ) SumSH
FROM tblEvents
WHERE Design = 'GoodDesign' AND Month = 'May' 
AND Unit_ID IN (SELECT Unit_ID FROM tblUnits WHERE City = 'NewYork')

此外，您的聚合列 Ev_Count 和 Event_Type 都使用条件：

Event_Type IN ('X', 'Y')

那么为什么不在 WHERE 子句中过滤这个条件并避免条件聚合：

SELECT
  COUNT(*) Ev_Count,
  SUM(Duration_Hrs) Duration,
  (
    SELECT SUM(Service_Hrs)
    FROM tblMetrics
    WHERE Month = 'May'
    AND Unit_ID IN (SELECT Unit_ID FROM tblUnits WHERE City = 'NewYork')
  ) SumSH
FROM tblEvents
WHERE Design = 'GoodDesign' AND Month = 'May' 
AND Event_Type IN ('X', 'Y')
AND Unit_ID IN (SELECT Unit_ID FROM tblUnits WHERE City = 'NewYork')