SQL Get sum of a column from another table by ID

Question

very inexperienced with SQL, and I've found myself needing to write a query. Hopefully you can help me understand how I'd go about this:

I have two tables.

• "table_requests" contains all requests, some of which are batches

• "table_pages" contains information for each page of a batch, connects to "table_requests" on the column "table_request_id"

In addition, "table_pages" has a numeric column "word_count" that lists a number for each page and a "table_request_id" column that can match to the PK of "table_requests".

For my query, I'd like to connect "table_requests" to "table_pages" on that matching column, and select everything from "table_requests" with an added column on the end that totals the "word_count" for each "table_request" (from all pages in "table_pages").

So far I have:

select tr.id, tr.creation_date, sum(tp.word_count) as total_wc
from table_requests tr 
join table_pages cp on tp.table_request_id = tr.id

Thank you all, let me know if there is any more information I can provide!

Answer 1

I think that the simplest approach is a correlated subquery:

select 
    tr.*,
    (
        select sum(tp.word_count) 
        from table_pages tp 
        where tp.table_request_id = tr.id
    ) total_wc
from table_requests tr

For performance with this query, make sure that you have an index on table_pages(table_request_id ) .