mysql - MySQL嵌套选择查询？

Question

好的，所以我有以下查询：

SELECT MIN(`date`), `player_name`
FROM `player_playtime`
GROUP BY `player_name`

然后我需要在以下查询中使用此结果：

SELECT DATE(`date`) , COUNT(DISTINCT  `player_name`)
FROM  `player_playtime /*Use previous query result here*/`
GROUP BY DATE( `date`) DESC LIMIT 60

我该怎么做呢？

Answer 1

您只需将第一个查询编写为子查询（派生表），在括号内，为其选择一个别名（t如下）并为列加上别名。

DISTINCT也可以安全地删除，因为内部使其GROUP BY冗余：

SELECT DATE(`date`) AS `date` , COUNT(`player_name`) AS `player_count`
FROM (
    SELECT MIN(`date`) AS `date`, `player_name`
    FROM `player_playtime`
    GROUP BY `player_name`
) AS t
GROUP BY DATE( `date`) DESC LIMIT 60 ;

由于COUNT现在很明显只计算派生表的行数，因此您可以将其替换为COUNT(*)并进一步简化查询：

SELECT t.date , COUNT(*) AS player_count
FROM (
    SELECT DATE(MIN(`date`)) AS date
    FROM player_playtime
    GROUP BY player_name
) AS t
GROUP BY t.date DESC LIMIT 60 ;