Question

Which is better in performance [Subquery] or [join]?

I have 3 tables related to each other, and i need to select data from one table that has some fields related to the other 2 tables, which one from the following 2 SQL statements is better from the view of performance :

select Table1.City, Table1.State, Table2.Name, Table1.Code, Table3.ClassName 
from Table1 inner join Table2 on Table1.EmpId = Table2.Id inner join Table3
on Table1.ClassId = Table3.Id where Table.Active = 1

OR

select City, State, (select Name from Table2 where Id = Table1.EmpId) as Name, Code,
(select ClassName from Table3 where Id = Table1.ClassId) as ClassName from Table1
where Active = 1

I have tried the execution plan but its statistics is not expressive to me because the current data is a test data not real one, so i can’t imagine the amount of data when tables are live of course they will be more than the test one.

Note : The Id field in Table2 and Table3 is primary key

Thanks in advance

Answer 1

The first approach, with joins, is by far faster. In second the query will be executed for each row. Some databases optimize nested queries into joins though.

Join vs. sub-query