如何在 MySQL 的 SELECT 语句中正确添加附加列？

Question

我想提取在两个不同的月份：三月和四月发生的“表演”中“教练”的出席人数（即 COUNT()）。 我设法创建了一个仅在一个月内收集该数字的查询。 另外，通过稍微修改查询，可以很容易地找到第二个月的数字。 但是如何将它们合并到一个包含两列的表中？

那么，鉴于下面的两个查询和结果表，如何将查询 2 的结果“附加”到查询 1 的结果中？ 换句话说，如何组合它们各自的 SELECT 语句？

我包含了 SQL 小提琴的链接，以防您需要它们。

先感谢您。

SQL小提琴

查询 1 ：

SELECT C.*, COUNT(CIS.idCoach) AS MarchNumOfShows
FROM Coach AS C
  LEFT JOIN 
    (
        CoachInShow AS CIS
     LEFT JOIN
        TVShow AS S
     ON S.idShow = CIS.idShow
    )
  ON C.idCoach = CIS.idCoach AND S.airDate LIKE '_____04___'
  GROUP BY C.idCoach

结果：

| idCoach |      name | surname | MarchNumOfShows |
|---------|-----------|---------|-----------------|
|       1 |   Stephen | Hawking |               5 |
|       2 |  Nicholas |    Cage |               7 |
|       3 | Sigourney |  Weaver |               6 |

查询 2 （最小差异，查询四月而不是三月）：

SELECT COUNT(CIS.idCoach) AS AprilNumOfShows
FROM Coach AS C
  LEFT JOIN 
    (
        CoachInShow AS CIS
     LEFT JOIN
        TVShow AS S
     ON S.idShow = CIS.idShow
    )
  ON C.idCoach = CIS.idCoach AND S.airDate LIKE '_____05___'
  GROUP BY C.idCoach

结果：

| AprilNumOfShows |
|-----------------|
|               8 |
|               7 |
|              10 |

通缉：

| idCoach |      name | surname | MarchNumOfShows | AprilNumOfShows |
|---------|-----------|---------|-----------------|-----------------|
|       1 |   Stephen | Hawking |               5 |               8 |
|       2 |  Nicholas |    Cage |               7 |               7 |
|       3 | Sigourney |  Weaver |               6 |              10 |

Answer 1

您非常接近，您错过的最后一步simply combine MarchNumOfShows and AprilNumOfShows with left join 。

像下面的代码（或查看Sql Fiddle ）：

SELECT C.idCoach, C.name, C.surname, COUNT(distinct CIS4.idShow) AS MarchNumOfShows
, COUNT(distinct CIS5.idShow) AS AprilNumOfShows
FROM Coach AS C
  LEFT JOIN 
    (
        CoachInShow AS CIS4
     LEFT JOIN
        TVShow AS S4
     ON S4.idShow = CIS4.idShow
    )
  ON C.idCoach = CIS4.idCoach AND S4.airDate LIKE '_____04___'
  LEFT JOIN 
    (
        CoachInShow AS CIS5
     LEFT JOIN
        TVShow AS S5
     ON S5.idShow = CIS5.idShow
    )
  ON C.idCoach = CIS5.idCoach AND S5.airDate LIKE '_____05___'
  GROUP BY C.idCoach;

下面是获得相同输出的另一种方法（或查看SQL Fiddle ）：

SELECT C.idCoach, C.name, C.surname, 
  sum(case when DATE_FORMAT(airDate,'%M')='April' then 1 else null end ) AS AprilNumOfShows, 
  sum(case when DATE_FORMAT(airDate,'%M')='May' then 1 else null end ) AS MayNumOfShows
FROM Coach AS C
  LEFT JOIN 
    (
        CoachInShow AS CIS
     LEFT JOIN
        TVShow AS S
     ON S.idShow = CIS.idShow
    )
  ON C.idCoach = CIS.idCoach 
  GROUP BY C.idCoach;

Answer 2

一种方法是使用案例：

select *,
  sum(case when airdate like "%03%" then 1 else 0 end) as March,
  sum(case when airdate like "%04%" then 1 else 0 end) as April
  ...