SQL GROUP BY和MAX

Question

I have the following table in my database:

-------------------------------------------
| value  | category |         date        |
-------------------------------------------
| 12.2   | A        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 13.3   | A        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 11.0   | B        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 11.2   | C        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 19.2   | C        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 10.4   | C        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------

And i'm trying to get the Maximum Value(12.2, 13.3,..) and the date correspondant to this value of each Category(A, B, C).

Like this for example:

-------------------------------------------
| value  | category |         date        |
-------------------------------------------
| 13.3   | A        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 11.0   | B        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------
| 19.2   | C        | yyyy-MM-dd HH:mm:ss | 
-------------------------------------------

I have followed this link SQL: How to select a max value for each group per day?

So I made this query:

SELECT MAX(value), category, date FROM myTable GROUP BY category;

But for some reason i don't know why it doesn't gave me what i'm looking for, it gave's me a wrong value of the DATE (DATE does not correspondant to the Max(value))!!!

Answer 1

You can do this using a sub qyery.

SELECT MAX(t.value), 
    t.category,
    (SELECT m.date FROM myTable as m 
     WHERE m.value = MAX(t.value) AND m.category = t.category) as Category
FROM myTable as t
GROUP BY category;

Answer 2

But for some reason i don't know why it doesn't gave me what i'm looking for, it gave's me a wrong value

The reason it didn't give you the correct values in this case:

SELECT MAX(value), category, date FROM myTable GROUP BY category;

is that, when you include a columns in the select clause in mysql that are not included in the group by nor an aggregate function like date , mysql selects an arbitrary values for it, so that it gives you the wrong values not the values that corresponds to the max values.

---

And i'm trying to get the Maximum Value(12.2, 13.3,..) and the date correspondant to this value of each Category(A, B, C).

To get the date s that are correspondant to the MAX(value) try this instead:

SELECT t1.*
FROM myTable AS t1
INNER JOIN
(
  SELECT MAX(value) AS MaxV, category
  FROM myTable AS t1
  GROUP BY category
) AS t2 ON  t2.MaxV     = t1.value
        AND t1.Category = t2.category;

The join to the subquery will ensure that the returned date value correspondant to the Max(value) and will eliminate the other values.

• SQL Fiddle Demo