Question

我有三张桌子：历史，视频和用户。以下两个查询为每个视频显示法国用户或德国用户的观看次数：

SELECT V.idVideo, COUNT(H.idVideo) AS nb_fr
FROM HISTORY H
INNER JOIN VIDEO V ON V.idVideo = H.idVideo
INNER JOIN USER U ON U.idUser = H.idUser
WHERE U.nationality = 'french'
GROUP BY H.idVideo
ORDER BY V.idVideo;

和

SELECT V.idVideo, COUNT(H.idVideo) AS nb_ge
FROM HISTORY H
INNER JOIN VIDEO V ON V.idVideo = H.idVideo
INNER JOIN USER U ON U.idUser = H.idUser
WHERE U.nationality = 'german'
GROUP BY H.idVideo
ORDER BY V.idVideo

但是如何将这两个查询结合起来只有一个？我想要这样的东西：

idVideo | nb_fr | nb_ge
-----------------------
    1   |   5   |   4
    2   |   3   |   6
    3   |   2   |   8
    4   |   3   |   3

Answer 1

使用case表达式进行条件聚合：

SELECT V.idVideo,
       COUNT(case when U.nationality = 'french' then H.idVideo end) AS nb_fr,
       COUNT(case when U.nationality = 'german' then H.idVideo end) AS nb_ge
FROM HISTORY H
INNER JOIN VIDEO V ON V.idVideo = H.idVideo
INNER JOIN USER U ON U.idUser = H.idUser
WHERE U.nationality in ('french', 'german')
GROUP BY V.idVideo
ORDER BY V.idVideo;

请注意对GROUP BY V.idVideo的更改，因为这是所选列。

Answer 2

您可以使用CASE EXPRESSION的条件聚合：

SELECT V.idVideo,
       COUNT(CASE WHEN  U.nationality = 'french' THEN 1 END) AS nb_fr,
       COUNT(CASE WHEN  U.nationality = 'german' THEN 1 END) AS nb_ge
FROM HISTORY H
INNER JOIN VIDEO V ON V.idVideo = H.idVideo
INNER JOIN USER U ON U.idUser = H.idUser
GROUP BY H.idVideo
ORDER BY V.idVideo

Answer 3

使用条件聚合：

SELECT H.idVideo,
       SUM(CASE WHEN U.nationality = 'french' THEN 1 ELSE 0 END) as nb_fr,
       SUM(CASE WHEN U.nationality = 'german' THEN 1 ELSE 0 END) as nb_ge
FROM HISTORY H INNER JOIN
     USER U 
     ON U.idUser = H.idUser
GROUP BY H.idVideo
ORDER BY H.idVideo;

另请注意，查询可以简化。可能不需要JOIN到VIDEO（假设idVideo是唯一的，这是一个非常合理的假设）。

此外，ORDER BY和GROUP BY应使用相同的列引用。

Answer 4

从Oracle 11.1开始，您可以使用PIVOT操作：

select idvideo, nb_fr, nb_ge
from   ( select h.idvideo, u.nationality
         from   history h inner join user u on h.iduser = u.iduser
       )
pivot  ( count(*) for nationality in ('french' as nb_fr, 'german' as nb_ge) )
order by idvideo
;

将两个选择查询合并为一个

4 个答案: