Difference between two similar queries that cause different results [closed] - sql

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I have a table tbl and I want to extract count of some data from it.
As simplified, I have two choices:
select count(if(c10=7,1,null)) as recordCount
from tbl
where cId in (
select max(cId)
from tbl
group by c37
);
and
select count(*) as recordCount
from (
select max(cId)
from tbl
where c10 = 7
group by c37
) t1;
The first query returns 151 and the second returns 156.
Why do these two queries have different results?

Answer 1

Select Count(if(c10=7,1,null)) as recordCount
from tbl
where cId in (select max(cId)
from tbl
group by c37);
In English -- for every grouping of c37 take the one with max id -- from records that match those ids if c10 = 7 then count them
Select Count(*) as recordCount
from (Select max(cId)
from tbl
where c10=7
group by c37) t1;
In English -- for every grouping of c37 with c10 is 7 take the max id -- count those.
I wrote the above to try and figure out what the queries are doing. Then I saw the issue as described below.
As for your question, it makes no sense the numbers would be different until you consider that max(cID) could be null in the second one but can't be in the first one. -> You can have null in a row in a table, but you can't have it when you use an IN statement.
To show this is the case run the following query
SELECT X
FROM (
Select max(cId) as X
from tbl
where c10=7
group by c37
) Z
WHERE X IS NULL
I'd expect you to get 4 rows back.
To get the same result for both queries use the following for the 2nd query
Select Count(X) as recordCount
from (Select max(cId) X
from tbl
where c10=7
group by c37) t1;