MySql And variables to exclude resultset - mysql

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)
)

Answer 2

From the other sample and it's comments, I would approach your query this way. The first query to pre-qualify if there are ANY "C = 1" values and store into a CCnt field. If none found, force the CCnt value to NULL. SECOND, do the same against the "B" column. Since each of these has a limit of 1, should be fast as long as it can FIND an entry of C=1 or B=1 respectively. if not, their values are NULL. Why do this way? The query is done ONCE per run of the query and the answer is done. It doesn't have to keep doing the query repeatedly as can be KILLER performance when used as sub-queries. Doing a count() with no group by will GUARANTEE as single row, so you won't have any Cartesian result that could result in duplicates.
Now that this preemptive querying is done, now add in your main table. At this point, the "HasCEntry.CCnt" value will either be a 1 or NULL.
"HasBEntry.CCnt" value will either be a 1 or NULL.
select
YT.*
from
( select IF( count(*) > 0, 1, NULL ) as CCnt
from YourTable
where C = 1
limit 1 ) HasCEntry,
( select IF( count(*) > 0, 1, NULL ) as BCnt
from YourTable
where B = 1
limit 1 ) HasBEntry,
YourTable YT
where
YT.C = 1
OR ( YT.B = 1 AND HasCEntry.CCnt IS NULL )
OR ( YT.A = 1 AND HasBEntry.BCnt IS NULL AND HasCEntry.CCnt IS NULL )
Now, following the WHERE clause. If there is even 1 record where C=1, then the
YT.C = 1 is all you need and its done... and the other "OR" conditions you don't care about, the record is coming along.
Now, say there are NO entries for C = 1. We know the "CCnt" value is NULL, so that will come into the first "OR" condition and testing against "B"... B Specifically = 1 AND the CCnt IS NULL... So, if there was an entry for "C=2" (3, 4 or any other value), we don't want it. We only want B = 1 AND C = NULL...
If C and B fail, then we fall into the "A" test. Likewise, A = 1 AND B is null AND C is NULL for same reasons as B above.
For a small table, you might not notice any significant performance difference from Shedal's answer. However, it COULD be a huge hit as your table grows.