Question

Please have a look at the tables below:

Customer Table:

ID
Name

Order Table:

ID
CustomerID

A customer can place 0,1 or many orders. Please have a look at the SQL query below:

SELECT Customer.* 
FROM Customer LEFT JOIN Order ON Customer.ID=Order.CustomerID 
WHERE CustomerID IS NULL

and

SELECT Customer.* 
FROM Customer LEFT JOIN Order ON Customer.ID=Order.CustomerID AND
      CustomerID IS NULL

Is there any difference between these two queries? When would a developer use one technique rather than the other?

I thought there would be other questions like this online, but I have not found the answer and hence the reason for the question.

Answer 1

In terms of result set and query plans the two are likely to produce identical results.

In that respect, they are the same.

If talking about readability, I would use the WHERE version, as the join condition is explicit and the WHERE clause is explicit to what results to include/exclude.