update with subquery - sql

Question

The code :
UPDATE tt_t_documents
SET t_Doc_header_ID = (SELECT
MIN(dh.Doc_header_ID)
FROM tt_t_documents td WITH (NOLOCK)
JOIN Doc_header dh WITH (NOLOCK)
ON dh.DH_doc_number = td.t_dh_doc_number
AND dh.DH_sub = 1
JOIN Pred_entry pe WITH (NOLOCK)
ON pe.Pred_entry_ID = dh.DH_pred_entry
JOIN Doc_type dt WITH (NOLOCK)
ON dty.Doc_type_ID = pe.PD_doc_type
AND dt.DT_mode = 5
HAVING COUNT(dh.Doc_header_ID) = 1);
I want to update my columns, but before that I also want to check if there is only one ID found.
The problem in this select is that I get more than one ID.
How can I write a query that updates each row and checks in the same query that there is only one id found?

Answer 1

I am guessing that you intend something like this:
update td
set t_Doc_header_ID = min_Doc_header_ID
from tt_t_documents td join
(select DH_doc_number, min(dh.Doc_header_ID) as min_Doc_header_ID
from Doc_header dh join
Pred_entry pe
on pe.Pred_entry_ID = dh.DH_pred_entry join
Doc_type dt
on dty.Doc_type_ID = pe.PD_doc_type and dt.DT_mode = 5
where dh.DH_doc_number = td.t_dh_doc_number and dh.DH_sub = 1
group by DH_doc_number
having count(dh.Doc_header_ID) = 1
) dh
on dh.DH_doc_number = td.t_dh_doc_number;
Using a join also means that you do not update the values where the condition does not match. If you use a left join, then the values will be updated to NULL (if that is your intention).

Answer 2

I'm not sure I believe you are getting more than one id back with that select given that you are doing a 'min' on it and have no group by. It should be only returning the lowest value for Doc_header_id.
To do what you are asking, first, you should have some way of joining to the tt_t_documents table in the update statement (eg. where td.id == tt_t_documents.id).
Second, you could re-write it to use the sub-query in the from. Something like:
update
tt_t_documents
set
t_Doc_header_ID = x.Doc_Header_id
from tt_t_documents join (
select td.id,
min(dh.Doc_header_ID)
from
tt_t_documents td
join Doc_header dh
on dh.DH_doc_number = td.t_dh_doc_number
and dh.DH_sub = 1
join Pred_entry pe
on pe.Pred_entry_ID = dh.DH_pred_entry
join Doc_type dt
on dty.Doc_type_ID = pe.PD_doc_type
and dt.DT_mode = 5
group by td.id
having
count(dh.Doc_header_ID) = 1
) x on tt_t_documents.id= x.id;
The syntax may not be perfect and I'm not sure how you want to find the doc_header_id but it would be something like this. The sub query would only return values with 1 doc_header_id). Not knowing the schema of your tables, this is as close as I can get.