CODEWITHSUNDEEP
mysqlgreatest-n-per-group
Jamie
Jamie

Reputation: 724

MySQL select unique rows in two columns with the highest value in one column

I have a basic table:

+-----+--------+------+------+
| id, | name,  | cat, | time |
+-----+--------+------+------+
|   1 | jamie  |    1 |  100 |
|   2 | jamie  |    2 |  100 |
|   3 | jamie  |    1 |   50 |
|   4 | jamie  |    2 |  150 |
|   5 | bob    |    1 |  100 |
|   6 | tim    |    1 |  300 |
|   7 | alice  |    4 |  100 |
+-----+--------+------+------+

I tried using the "Left Joining with self, tweaking join conditions and filters" part of this answer: SQL Select only rows with Max Value on a Column but some reason when there are records with a value of 0 it breaks, and it also doesn't return every unique answer for some reason.

When doing the query on this table I'd like to receive the following values:

+-----+--------+------+------+
| id, | name,  | cat, | time |
+-----+--------+------+------+
|   1 | jamie  |    1 |  100 |
|   4 | jamie  |    2 |  150 |
|   5 | bob    |    1 |  100 |
|   6 | tim    |    1 |  300 |
|   7 | alice  |    4 |  100 |
+-----+--------+------+------+

Because they are unique on name and cat and have the highest time value.

The query I adapted from the answer above is:

SELECT a.name, a.cat, a.id, a.time
FROM data A
INNER JOIN (
    SELECT name, cat, id, MAX(time) as time
    FROM data
    WHERE extra_column = 1
    GROUP BY name, cat
) b ON a.id = b.id AND a.time = b.time

Upvotes: 1

Views: 193

Answers (3)

hoda
hoda

Reputation: 11

It is very simple

SELECT MAX(TIME),name,cat FROM table name group by cat

Upvotes: 0

xQbert
xQbert

Reputation: 35323

The issue here is that ID is unique per row you can't get the unique value when getting the max; you have to join on the grouped values instead.

SELECT a.name, a.cat, a.id, a.time
FROM data A
INNER JOIN (
    SELECT name, cat, MAX(time) as time
    FROM data
    WHERE extra_column = 1
    GROUP BY name, cat
) b ON A.Cat = B.cat and A.Name = B.Name AND a.time = b.time

Think about it... So what ID is mySQL returning form the Inline view? It could be 1 or 3 and 2 or 4 for jamie. Hows does the engine know to pick the one with the max ID? it is "free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. " it could pick the wrong one resulting in incorrect results. So you can't use it to join on.

https://dev.mysql.com/doc/refman/5.0/en/group-by-handling.html

Upvotes: 3

fthiella
fthiella

Reputation: 49049

If you want to use a self join, you could use this query:

SELECT
  d1.*
FROM
  date d1 LEFT JOIN date d2
  ON d1.name=d2.name
     AND d1.cat=d2.cat
     AND d1.time<d2.time
WHERE
  d2.time IS NULL

Upvotes: 3

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