group Items by column and order by other column - mysql

Question

I have table as below , I want to take latest rating for the client
basically user whenever updates rating, count will be incremented and a entry will be made in table. Table goes as below
-----------------------------------------------------
|_id| name | client_id | user_id | rating | count |
-----------------------------------------------------
|1 | Four | 1 | 1 | 4 | 1 |
|2 | three | 1 | 1 | 3 | 2 |
|3 | two | 1 | 1 | 2 | 3 |
|4 | five | 1 | 1 | 5 | 4 |
|5 | two | 1 | 2 | 2 | 1 |
|6 | three | 1 | 2 | 3 | 2 |
|7 | two | 2 | 1 | 2 | 1 |
|8 | three | 2 | 1 | 3 | 2 |
-----------------------------------------------------
For rating of client_id 1 I want out put like
-----------------------------------------------------
|_id| name | client_id | user_id | rating | count |
-----------------------------------------------------
|4 | five | 1 | 1 | 5 | 4 |
|6 | three | 1 | 2 | 3 | 2 |
-----------------------------------------------------
so far I tried SELECT * FROM test
where client_id = 1 group by client_id order by count desc;
but not getting expected result, any help??

Answer 1

You can use left join on the same table as
select t1.* from test t1
left join test t2 on t1.user_id = t2.user_id
and t1.client_id = t2.client_id
and t1._id < t2._id
where
t2._id is null
and t1.client_id = 1
order by t1.`count` desc;
Using un-correlated subquery you may do as
select t1.* from test t1
join (
select max(_id) as _id,
client_id,
user_id
from test
where client_id = 1
group by client_id,user_id
)t2
on t1._id = t2._id
and t1.client_id = t2.client_id
order by t1.`count` desc;
UPDATE : From the comment how to join another table into above , for this here is an example
mysql> select * from users ;
+------+------+
| _id | name |
+------+------+
| 1 | AAA |
| 2 | BBB |
+------+------+
2 rows in set (0.00 sec)
mysql> select * from test ;
+------+-------+-----------+---------+--------+-------+
| _id | name | client_id | user_id | rating | count |
+------+-------+-----------+---------+--------+-------+
| 1 | four | 1 | 1 | 4 | 1 |
| 2 | three | 1 | 1 | 3 | 2 |
| 3 | two | 1 | 1 | 2 | 3 |
| 4 | five | 1 | 1 | 5 | 4 |
| 5 | two | 1 | 2 | 2 | 1 |
| 6 | three | 1 | 2 | 3 | 2 |
| 7 | two | 2 | 1 | 2 | 1 |
| 8 | three | 2 | 1 | 3 | 2 |
+------+-------+-----------+---------+--------+-------+
select t1.*,u.name from test t1
join users u on u._id = t1.user_id
left join test t2 on t1.user_id = t2.user_id
and t1.client_id = t2.client_id
and t1._id < t2._id
where
t2._id is null
and t1.client_id = 1
order by t1.`count` desc;
Will give you
+------+-------+-----------+---------+--------+-------+------+
| _id | name | client_id | user_id | rating | count | name |
+------+-------+-----------+---------+--------+-------+------+
| 4 | five | 1 | 1 | 5 | 4 | AAA |
| 6 | three | 1 | 2 | 3 | 2 | BBB |
+------+-------+-----------+---------+--------+-------+------+
Note that the join to users table is inner join and this will require all the user to be preset in users table which are in test table
If some users are missing in the users table then use left join this will have null values for the data selected from users table.

Answer 2

You may try something like
select _id, name, client_id, user_id, rating, max(count)
from clients
group by client_id

Answer 3

Try it
SELECT * FROM test
where client_id = 1
group by user_id
order by count desc