Question

I need to use t-sql to query two tables. The first table is Books. The second table is Authors. For each Book record there could be multiple child Author records. I want to write a query that only returns the first Author record found for the current Book record. There are hundreds of thousands of records in the tables so I need the query to be efficient.

select a.FirstName, a.LastName, b.BookName
from Books b
left join 
(
    select TOP 1 t.BookID, t.FirstName, t.LastName 
    from Authors t
) a 
    on a.BookID = b.BookID
where b.BookClassification = 2

This query is not right. I only want to select the top 1 record in the Authors which match the BookID. How can I get the results I am looking for?

Answer 1

You were close:

select a.FirstName, a.LastName, b.BookName
from Books b
outer apply 
(
    select TOP 1 t.BookID, t.FirstName, t.LastName 
    from Authors t
    WHERE t.BookID = b.BookID
    -- uncomment the next line to control which author to prefer
    -- ORDER BY t.<someColumn>...
) a 
where b.BookClassification = 2

Though it seems odd to me that Authors would be a child of Books… 🙂