how to use a field from a main query within the WHERE of a sub query

Question

been a long time reader so thanks for all the help thus far and sorry if I make any noob mistakes creating this question!

I am trying to build a query that is already quite complex (for me anyway) and I need to have one of the fields for each row be a single result from another query using a field from the main query as it's WHERE statement.

I have tried using variables but it seems you cant set these after the main select, makes sense I guess but at the time I was thinking of the select as working like a foreach loop as it collects the rows.

It would take a very long time to explain the whole structure of the databases (the tables are provided to me and need to stay how they are) but I will try and capture the query structure with the following example.

Select

a.field1,
a.field2,
b.field1,
b.field2,
c.field3,
c.field3,
( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) as field7

FROM
All the tables and joins
WHERE {some stuff}

Bottom line is that the main query works in isolation perfectly and the sub query also works fine on it's own when I feed it a faked a.field1 value

But together the query runs but just returns NULL for the field7 column.

I started out with trying to set a variable to the a.field1 value just after the ( for the sub query but this just causes intellisense to go nuts on all the table names above.

Any pointers would be much appreciated!

Answer 1

AFAIK, you can't use them directly.

Your options are repeating the subselect:

Select
a.field1,
a.field2,
b.field1,
b.field2,
c.field3,
c.field3,
( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) as field7

FROM
All the tables and joins
WHERE 
( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) = somevalue;

Or making everything a sub select:

select result.* from
(Select
    a.field1,
    a.field2,
    b.field1,
    b.field2,
    c.field3,
    c.field3,
    ( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) as field7

    FROM
    All the tables and joins
    WHERE {some stuff}) as result
where result.field7 = somevalue

Answer 2

Would this do it?

Select a.field1,
    a.field2,
    b.field1,
    b.field2,
    c.field3,
    c.field3,
    aa.Field1 as field7

FROM All the tables and joins
LEFT JOIN ( select aa.field1,
                   zz.Field3
            from zz inner join aa on zz.field2 = aa.field3) aa
ON aa.field3 = a.field1 
WHERE {some stuff}