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

SELECT a.*
FROM test a
NATURAL JOIN
(
    SELECT c.Head, c.Id, MIN(c.IntTime) AS IntTime
    FROM test c
    NATURAL JOIN
    (
        SELECT Id, MIN(IntTime) AS IntTime
        FROM test
        GROUP BY Id
    ) d
    GROUP BY c.Head
) b
ORDER BY a.Location

Answer 2

I am not sure how your sample result arrived at Location = 7 for ARMH, for instance. Try this...

SELECT MIN(Location), Head, Id, MIN(IntTime)
FROM test
GROUP BY Head, Id

Answer 3

If I understand correctly, you want the rows that have the smallest int/time values by head and id. I am not seeing what the second "group by head" is getting you.

The following does what I think you want:

select t.*
from t join
     (select head, id, min(intTime) as min_intTime
      from t
      group by head, id
     ) tsum
     on tsum.head = t.head and
        tsum.id = t.id and
        tsum.min_intTime = t.intTime

You might just want the smallest intTime for all "head" values. If so, the query looks like:

select t.*
from t join
     (select head, min(intTime) as min_intTime
      from t
      group by head
     ) tsum
     on tsum.head = t.head and
        tsum.min_intTime = t.intTime

Answer 4

Use UNION syntax?

(SELECT Location, Head, Id, MIN(IntTime) FROM test 
GROUP BY Id)
UNION
(SELECT Location, Head, Id, MIN(IntTime) FROM test2 
GROUP BY Head)

Answer 5

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).