MySQL嵌套选择查询？

Question

Ok, so I have the following query:

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

I then need to use this result inside the following query:

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?

Answer 1

You just need to write the first query as a subquery (derived table), inside parentheses, pick an alias for it ( t below) and alias the columns as well.

The DISTINCT can also be safely removed as the internal GROUP BY makes it redundant:

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 COUNT is now obvious that is only counting rows of the derived table, you can replace it with COUNT(*) and further simplify the query:

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 ;