Question

I have a doubt on how variables work in mysql. As I read in their web looks like setting a variable will become visible to the next row.

My table is like:

A     B     C      N
1    NULL   NULL   4
1    NULL   NULL   4 
1     1     NULL   4 
1     1     NULL   4
1     1     1      4
1     1     1      4

What I want is to return only the rows with C = 1. If no rows then return B = 1 and C is NULL if no rows A = 1 and B is NULL and C is NULL.

My idea was:

select N as number,
       @var_c := case when (C = 1) then 1 else -1 end as myc,
       @var_b := case when (@var_c < 0 and B = 1) then 1 else -1 end as myB,
       @var_c := case when (@var_a < 0 and var_b < 0 and C = 1) then 1 else -1 end as myC

from (select @var_a := -1) r_a,
     (select @var_b := -1) r_b,
     (select @var_c := -1) r_c,
     (select A, B, C, N from my_table order by A desc, B desc, C desc) rows

It should (I want to) return

number     myA    myB    myC
  4         -1      -1    1
  4         -1      -1    1
  4         -1      -1    -1
  4         -1      -1    -1
  4         -1      -1    -1
  4         -1      -1    -1

With this and having myA > 0 or myB > 0 or myC > 0 would work.

But it is returning

number    myA     myB    myC
  4         1      -1    -1
  4         1      -1    -1
  4         -1      -1    1
  4         -1      -1    1
  4         -1      1    -1
  4         -1      1    -1

Shouldn’t Mysql keep the vars across the rows?

Regards.

Answer 1

First of all, do not use variables like this. You shouldn’t expect that you can set a variable in a FROM clause and then access the value in the SELECT clause. That’s not how variables work. Normally you should only use a variable in one place in the query, otherwise you can get undeterministic results.

Second of all, why don’t you just issue three different queries? First for A = 1 and B is NULL and C is NULL, and if it doesn’t return any rows, issue a query with the second condition set. And so forth.

And if you ultimately want to issue just a single query, you can try this:

SELECT N as number
FROM my_table
WHERE IF(EXISTS (SELECT A FROM my_table WHERE A = 1 and B is NULL and C is NULL),
    A = 1 and B is NULL and C is NULL,
    IF(EXISTS (SELECT A FROM my_table WHERE B = 1 and C is NULL), B = 1 and C is NULL, C = 1))

But it’s very likely to kill performance. So better just use three queries instead of one.

UPD: There’s another (yet similar) approach:

SELECT N as number
FROM my_table
WHERE (
  A = 1
  AND B is NULL
  AND C is NULL
) OR (
  B = 1
  AND C is NULL
  AND NOT EXISTS (SELECT A FROM my_table WHERE A = 1 and B is NULL and C is NULL)
) OR (
  C = 1
  AND NOT EXISTS (SELECT A FROM my_table WHERE A = 1 and B is NULL and C is NULL)
  AND NOT EXISTS (SELECT A FROM my_table WHERE B = 1 and C is NULL)
)