Use of GROUP BY twice in MySQL

Question

My table looks like this.

Location    Head    Id  IntTime
1           AMD     1   1
2           INTC    3   3
3           AMD     2   2
4           INTC    4         


        

Answer 1

This query returns the exact final results you're looking for (example):

SELECT `final`.*
FROM `tableName` AS `final`
JOIN (
    SELECT `thead`.`Id`, `Head`, MIN(`intTime`) AS `min_intTime`
    FROM `tableName` AS `thead`
    JOIN (
        SELECT `Id`, MIN(intTime) as min_intTime
        FROM `tableName` AS `tid`
        GROUP BY `Id`
    ) `tid`
    ON `tid`.`Id` = `thead`.`Id`
    AND `tid`.`min_intTime` = `thead`.`intTime`
    GROUP BY `Head`
) `thead`
ON `thead`.`Head` = `final`.`Head`
AND `thead`.`min_intTime` = `final`.`intTime`
AND `thead`.`Id` = `final`.`Id`

How it works

The innermost query groups by Id, and returns the Id and corresponding MIN(intTime). Then, the middle query groups by Head, and returns the Head and corresponding Id and MIN(intTime). The final query returns all rows, after being narrowed down. You can think of the final (outermost) query as a query on a table with only the rows you want, so you can do additional comparisons (e.g. WHERE final.intTime > 3).