Select row where row number is equal to some value from other table

Question

I have two tables: game and tasks

Game looks like this:

| step | manualTaskCounter | autoTaskCounter | (and other)
----------------------------------------------------------
|  1   |      3            | 1               | ...
----------------------------------------------------------

Tasks looks like this:

| id | taskType | taskContent |
-------------------------------
|  1 |    M     | abc         |
|  2 |    M     | cde         |
|  3 |    A     | efg         |
|  4 |    M     | jpq         |

Since tasks holds both, manual (with M taskType) and automatic (A) tasks I want to select. My API holds two variables: mTaskCounter and aTaskCounter. for example if mTaskCounter = 3 I want to select 3rd row of type manualTask from tasks. Since it is in fact row with id = 4 I can not use id in WHERE clause.

What I already achieved is:

SELECT
    id,
    taskType,
    taskContent,
    (@row:=@row + 1) as rowNumber,
    g.manualTaskCounter as mTaskCounter
FROM
    tasks t,
    (SELECT @ROW:=0) AS r,
    (SELECT manualTaskCounter FROM game) AS g
WHERE
    g.manualTaskCounter = rowNumber

This says "unknown column 'rowNumber' in where clause

I also tried to use LEFT JOIN:

SELECT
    id,
    taskType,
    taskContent,
    (@row:=@row + 1) as rowNumber,
    g.manualTaskCounter as mTaskCounter
FROM
    tasks t,
    (SELECT @ROW:=0) AS r
LEFT JOIN
    `game` g ON g.manualTaskCounter = rowNumber

Same result. It's been a while since I used mysql everyday and dont know how to fix it. I also think to make two tables - manualTasks and autoTasks instead of tasks so it qould solve the problem by common select taskContent from autoTasks a LEFT JOIN game ON a.id = game.autoTaskCounter

Answer 1

For approaching your goal, first you will need to make derived tables for both manual and automatic tasks. Next queries will made those tables adding up the row number too:

Table With Manual Tasks

SELECT
    t.id,
    t.taskType,
    t.taskContent,
    (@row_num := @row_num + 1) AS rowNum
FROM
    tasks AS t
CROSS JOIN
    (SELECT @row_num := 0) AS r
WHERE
    taskType = 'M'

Table With Automatic Tasks

SELECT
    t.id,
    t.taskType,
    t.taskContent,
    (@row_num := @row_num + 1) AS rowNum
FROM
    tasks AS t
CROSS JOIN
    (SELECT @row_num := 0) AS r
WHERE
    taskType = 'A'

Now, all you need to do is join those derived tables with the game table on the adequate columns:

Select manual task number X using the manualTaskCounter field

SELECT
    mTasks.*
FROM
    game AS g
INNER JOIN
    ( SELECT
          t.id,
          t.taskType,
          t.taskContent,
          (@row_num := @row_num + 1) AS rowNum
      FROM
          tasks AS t
      CROSS JOIN
          (SELECT @row_num := 0) AS r
      WHERE
           taskType = 'M' ) AS mTasks ON mTasks.rowNum = g.manualTaskCounter

Select automatic task number X using the autoTaskCounter field

SELECT
    aTasks.*
FROM
    game AS g
INNER JOIN
    ( SELECT
          t.id,
          t.taskType,
          t.taskContent,
          (@row_num := @row_num + 1) AS rowNum
      FROM
          tasks AS t
      CROSS JOIN
          (SELECT @row_num := 0) AS r
      WHERE
           taskType = 'A' ) AS aTasks ON aTasks.rowNum = g.autoTaskCounter

Check the next online example:

DB Fiddle Example