Limiting joined table

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Hey guys :-)

I want to improve my MySQL knowledge and have given me an weird requirement.
So please don't laugh ;-)

Target : show the last three point-entries for each user in several rows

There are two tables like:

table user:
user_id | name
1 | first_name
2 | second_name
3 | third_name

and

table point:
user_id | points
1 | 10
1 | 20
1 | 30
1 | 40
2 | 15
2 | 25
2 | 35
2 | 45
3 | 11
3 | 22
3 | 33

The result should look like:

user_id | name | points
1 | first_name | 40
1 | first_name | 30
1 | first_name | 20
2 | second_name | 45
2 | second_name | 35
2 | second_name | 25
3 | third_name | 33
3 | third_name | 22
3 | third_name | 11

I tried it with my basic joining- and subquery knowledge, but I ran into several MySql errors.
Limiting is not allowed with “IN” and so on.

What would you do, to solve this?

Posted 19-Nov-15 2:04am

expo82

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PIEBALDconsult 19-Nov-15 8:06am

"What would you do, to solve this?" I'd use SQL Server instead.

expo82 19-Nov-15 17:52pm

I like this answer and I will believe you.

But for now there is one problem:
"I want to improve my MySQL knowledge and have given me an weird requirement." ;-)

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CHill60 · Accepted Answer · 2015-11-25T06:45:00

What you are after is ranking within a category, where each name is the "category" and the points will be ranked for each name.

I found a neat trick on this post[^] which I used in this query

SQL

SET @prev := null;

SET @cnt := 0;

CREATE TEMPORARY TABLE IF NOT EXISTS table2 AS 
(
  SELECT U.user_id, user_name, points, 
      IF(@prev <> U.user_id, @cnt := 1, @cnt := @cnt + 1) AS rank, 
      @prev := U.user_id
  FROM user U
  JOIN point P ON U.user_id = P.user_id
  ORDER BY U.user_id, points DESC;
);
select user_id, user_name, points from table2 where rank < 4;

Sorry - sqlfiddle crashed on me so I haven't been able to test it fully.

Points to note - I've ordered the query by user_id ascending but points Descending as the points values seemed to increase. I haven't tried the query without the order on points. If there was a date/time when the points were gained then you could use that to get "the last 3 points" otherwise you might want to have a play around with emulating the sql Row_number function[^]