MySQL MySQL嵌套选择查询？
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MySQL Nested Select Query?
提问by T I
Ok, so I have the following query:
好的，所以我有以下查询：
SELECT MIN(`date`), `player_name`
FROM `player_playtime`
GROUP BY `player_name`
I then need to use this result inside the following query:
然后我需要在以下查询中使用这个结果：
SELECT DATE(`date`) , COUNT(DISTINCT `player_name`)
FROM `player_playtime /*Use previous query result here*/`
GROUP BY DATE( `date`) DESC LIMIT 60
How would I go about doing this?
我该怎么做呢？
回答by T I
You just need to write the first query as a subquery (derived table), inside parentheses, pick an alias for it (tbelow) and alias the columns as well.
您只需要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）并为列设置别名。
The DISTINCTcan also be safely removed as the internal GROUP BYmakes it redundant:
该DISTINCT也可以安全删除，因为内部GROUP BY使得冗余：
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 ;
Since the COUNTis now obvious that is only counting rows of the derived table, you can replace it with COUNT(*)and further simplify the query:
由于COUNT现在很明显，它只计算派生表的行，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 ;
