将两个选择查询合并为一个

Question

我有三个表: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;

请注意更改为GROUP BY V.idVideo,因为这是所选列.

Answer 2

您可以使用条件聚合执行以下操作CASE EXPRESSION:

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