MySQL：在分组数据上使用 "inner join" 和 "count" 创建视图

Question

我的数据库包含一个包含许多不同数据列的 table。以下简单表示显示了我当前感兴趣的列：

Table 播放列表：

id       metadata                   lastplayed
===============================================
1        All Night                  1571324631
2        Perfect Replacement        1571324767
3        One More Day               1571324952
4        Stay Awake                 1571325184
5        Perfect Replacement        1571325386
6        All Night                  1571325771
7        Close Enemies              1571326422

我已经有了一个对元数据进行分组的视图，因此我可以看到歌曲的所有单次出现以及它们上次播放的时间（纪元秒）。

查看 'Music'（期望的结果）：

id       metadata                   lastplayed      count (*)
==============================================================
3        One More Day               1571324952      1
4        Stay Awake                 1571325184      1
5        Perfect Replacement        1571325386      2
6        All Night                  1571325771      2
7        Close Enemies              1571326422      1

“计数”列在视图中尚不存在，我想通过创建视图的现有 SQL 脚本将其包括在内：

CREATE VIEW `Music` AS
SELECT 
  t1.`id`,
  t1.`metadata`,
  t1.`lastplayed`
FROM Playlist t1
INNER JOIN
(
    SELECT `metadata`, MAX(`lastplayed`) AS `timestamp`
    FROM Playlist
    GROUP BY `metadata`
) t2
    ON t1.`metadata` = t2.`metadata` AND t1.`lastplayed` = t2.`timestamp`
ORDER BY t1.`Id` ASC

所以现在我运行陷入了将我的 COUNT(metadata) AS count 行放在何处以及如何放置的问题，以获得所需的结果。当我将它添加到顶部 SELECT 行时，table 减少为包含一首歌曲和所有数据行数的单个数据行。

Answer 1

放在里面select

CREATE VIEW `Music` AS
SELECT 
  t1.`id`,
  t1.`metadata`,
  t2.`lastplayed`,
  t2.count
FROM Playlist t1
INNER JOIN
(
    SELECT `metadata`, MAX(`lastplayed`) AS `timestamp`, COUNT(*) AS count
    FROM Playlist
    GROUP BY `metadata`
) t2
    ON t1.`metadata` = t2.`metadata` AND t1.`lastplayed` = t2.`timestamp`
ORDER BY t1.`Id` ASC

Answer 2

您只需要通过按 metadata 列

分组进行简单聚合

CREATE OR REPLACE VIEW `Music` AS
SELECT MAX(id) AS id, `metadata`, MAX(`lastplayed`) AS lastplayed, COUNT(*) AS count 
  FROM Playlist
 GROUP BY `metadata`

MySQL：在分组数据上使用 "inner join" 和 "count" 创建视图

MySQL: Create view with "inner join" and "count" on grouped data

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