Question

I have this as one of the fields in my select statement:

select
      ...some fields
            (select ISNULL( TagNames, '') from  TagNames_CTE as tagNames 
         where  Content.ID = tagNames.EntryID) as tags 
from  SomeTable

I noticed that even when I’m using ISNULL and telling it to replace those with empty string, I’m still getting NULL back for some records. It’s not replacing them with an empty string and I don’t see why.

Answer 1

I think you’ll find that the subselect actually returns NULL in those cases where no rows are found.

In cases where a row is found in TagNames_CTE where the where clause fires and TagNames is NULL, it will get converted to '' by the function.

But where no rows are found at all, the function is not called (since there are no rows for it to work its magic on) and the result of the overall subselect is NULL, because it has to return something in that column for the outer select.

An easy way to check is to run the subquery (without ISNULL()) on its own and see if it returns a row containing NULL or no rows at all – try some simple queries along the lines of:

select 1,
       (select isnull (null,2) from dummytable where 1 = 0)
from dummytable

and:

select 1,
       (select isnull (null,2) from dummytable where 1 = 1)
from dummytable

You’ll probably find that the former gives you 1, null while the latter gives you 1,2. Having tested that in MySQL with the following statement, you can see that this is most likely correct.

> create table xyzzy (plugh integer);

> insert into xyzzy values (42):

> select 1,(select ifnull (null, 2) from xyzzy where 1 = 0) from xyzzy;
1 NULL

> select 1,(select ifnull (null, 2) from xyzzy where 1 = 1) from xyzzy;
1 2