Question

I have a table that has records

id_queue | user_id | id_book | status

   69    |    5    |    4    |   1

   133   |    3    |    4    |   2

   142   |    1    |    4    |   0

I want a query that will give me this result

id_queue | id_queue

   69    |    142    

   133   |    null 

I’ve tried something like this

SELECT s1.`id_queue`,s2.`id_queue` FROM `second` as s1
LEFT JOIN `second` as s2 ON s2.`book_id`=4 AND s2.`status` IN (0)
WHERE s1.`book_id`=4 AND s1.`status` IN (1,2,4) 

but it keeps bringing me this result.

id_queue | id_queue

   69    |    142    

   133   |    142

I think this is because I don’t have anything identical for conditions. What can I do?

Answer 1

The issue is that you are joining on the book_id not the status, so you will always have a matching row.

If you want to hide the second instance of the second id_queue then you can use user defined variables:

select Queue1,
  case when seq = 1 then queue2 else null end Queue2
from
(
  select 
    Queue1,
    Queue2,
    @row:=(case when @prev=Queue2 then @row else 0 end) +1 as Seq,
    @prev:=Queue2 
  from
  (
    SELECT 
      s1.`id_queue` Queue1,
      s2.`id_queue` Queue2
    FROM `second` as s1
    LEFT JOIN `second` as s2 
      ON s1.`id_book` = s2.`id_book`
      AND s2.`status` = 0
    WHERE s1.`id_book`=4 
      AND s1.`status` IN (1,2,4)
  ) src, (SELECT @row:=0, @prev:=null) r
  order by Queue1, Queue2
) s1
order by Queue1, Queue2

See SQL Fiddle with Demo

The result is:

| QUEUE1 | QUEUE2 |
-------------------
|     69 |    142 |
|    133 | (null) |