Determine Rank based on Multiple Columns in MySQL

In a Derived Table (subquery inside the FROM clause), we order our data such that all the rows having same user_id values come together, with further sorting between them based on game_detail in Descending order.

Now, we use this result-set and use conditional CASE..WHEN expressions to evaluate the row numbering. It will be like a Looping technique (which we use in application code, eg: PHP). We would store the previous row values in the User-defined variables, and then check the current row’s value(s) against the previous row. Eventually, we will assign row number accordingly.

Edit: Based on MySQL docs and @Gordon Linoff’s observation:

The order of evaluation for expressions involving user variables is
undefined. For example, there is no guarantee that SELECT @a, @a:=@a+1
evaluates @a first and then performs the assignment.

We will need to evaluate row number and assign the user_id value to @u variable within the same expression.

SET @r := 0, @u := 0; 
SELECT
  @r := CASE WHEN @u = dt.user_id 
                  THEN @r + 1
             WHEN @u := dt.user_id /* Notice := instead of = */
                  THEN 1 
        END AS user_game_rank, 
  dt.user_id, 
  dt.game_detail, 
  dt.game_id 

FROM 
( SELECT user_id, game_id, game_detail
  FROM game_logs 
  ORDER BY user_id, game_detail DESC 
) AS dt 

Result

| user_game_rank | user_id | game_detail | game_id |
| -------------- | ------- | ----------- | ------- |
| 1              | 6       | 260         | 11      |
| 2              | 6       | 100         | 10      |
| 1              | 7       | 1200        | 10      |
| 2              | 7       | 500         | 11      |
| 3              | 7       | 260         | 12      |
| 4              | 7       | 50          | 13      |

View on DB Fiddle

---

An interesting note from MySQL Docs, which I discovered recently:

Previous releases of MySQL made it possible to assign a value to a
user variable in statements other than SET. This functionality is
supported in MySQL 8.0 for backward compatibility but is subject to
removal in a future release of MySQL.

Also, thanks to a fellow SO member, came across this blog by MySQL Team: https://mysqlserverteam.com/row-numbering-ranking-how-to-use-less-user-variables-in-mysql-queries/

General observation is that using ORDER BY with evaluation of the user variables in the same query block, does not ensure that the values will be correct always. As, MySQL optimizer may come into place and change our presumed order of evaluation.

Best approach to this problem would be to upgrade to MySQL 8+ and utilize the Row_Number() functionality:

Schema (MySQL v8.0)

SELECT user_id, 
       game_id, 
       game_detail, 
       ROW_NUMBER() OVER (PARTITION BY user_id 
                          ORDER BY game_detail DESC) AS user_game_rank 
FROM game_logs 
ORDER BY user_id, user_game_rank;

Result

| user_id | game_id | game_detail | user_game_rank |
| ------- | ------- | ----------- | -------------- |
| 6       | 11      | 260         | 1              |
| 6       | 10      | 100         | 2              |
| 7       | 10      | 1200        | 1              |
| 7       | 11      | 500         | 2              |
| 7       | 12      | 260         | 3              |
| 7       | 13      | 50          | 4              |

View on DB Fiddle