How to do operations between a column and a subquery - sql

Question

I would like to know how I can do operations between a column and a subquery, what I want to do is add to the field Subtotal what was obtained in the subquery Impuestos, the following is the query that I am using for this case.
Select
RC.PURCHID;
LRC.VALUEMST as 'Subtotal',
isnull((
select sum((CONVERT(float, TD1.taxvalue)/100)*LRC1.VALUEMST ) as a
FROM TAXONITEM TOI1
inner join TAXDATA TD1 ON (TD1.TAXCODE = TOI1.TAXCODE and RC.DATAAREAID = TD1.DATAAREAID)
inner join TRANS LRC1 on (LRC1.VEND = RC.RECID)
WHERE TOI1.TAXITEMGROUP = PL.TAXITEMGROUP and RC.DATAAREAID = TOI1.DATAAREAID
), 0) Impuestos
from VEND RC
inner join VENDTABLE VTB on VTB.ACCOUNTNUM = RC.INVOICEACCOUNT
inner join TRANS LRC on (LRC.VEND = RC.RECID)
inner join PURCHLINE PL on (PL.LINENUMBER =LRC.LINENUM and PL.PURCHID =RC.PURCHID)
where year (RC.DELIVERYDATE) =2021 and RC.PURCHASETYPE =3 order by RC.PURCHID;
Hope someone can give me some guidance when doing operations with subqueries.

Answer 1

A few disjointed facts that may help:
When a SELECT statement returns only one row with one column, you can enclose that statement in parenthesis and use it as a plain value. In your case, let's say that select sum(......= TOI1.DATAAREAID returns 500. Then, your outer select's second column is equivalent to isnull(500,0)
You mention in your question "subquery Impuestos". Keep in mind that, although you indeed used a subquery as we mentioned earlier, by the time it was enclosed in parentheses it is not treated as a subquery (more accurately: derived table), but as a value. Thus, the "Impuestos" is only a column alias at this point
I dislike and avoid subqueries before the from, makes things much harder to read. Here is a solution with apply which will keep your code mostly intact:
Select
RC.PURCHID,
LRC.VALUEMST as 'Subtotal',
isnull(subquery1.a, 0) as Impuestos
from VEND RC
inner join VENDTABLE VTB on VTB.ACCOUNTNUM = RC.INVOICEACCOUNT
inner join TRANS LRC on (LRC.VEND = RC.RECID)
inner join PURCHLINE PL on (PL.LINENUMBER =LRC.LINENUM and PL.PURCHID =RC.PURCHID)
outer apply
(
select sum((CONVERT(float, TD1.taxvalue)/100)*LRC1.VALUEMST ) as a
FROM TAXONITEM TOI1
inner join TAXDATA TD1 ON (TD1.TAXCODE = TOI1.TAXCODE and RC.DATAAREAID = TD1.DATAAREAID)
inner join TRANS LRC1 on (LRC1.VEND = RC.RECID)
WHERE TOI1.TAXITEMGROUP = PL.TAXITEMGROUP and RC.DATAAREAID = TOI1.DATAAREAID
) as subquery1
where year (RC.DELIVERYDATE) =2021 and RC.PURCHASETYPE =3 order by RC.PURCHID;