Question

I have a database with customer information, orders, etc. I need to run a query that returns all customers who have not placed an order at all.

Relevant tables: login and orders

Relevant Columns: login.loginID, login.loginFirstName, login.loginLastName, login.loginEmailAddress AND orders.OrderuserID

So essentially – in psuedocode: compare table login, column loginID for matches in the orders table under orders.OrderUserID. If no match exists (as in no orders placed) then output the users First Name, Last Name and Email address.

I have been racking my brain but having some real issues with the language. I’m a big time N00B when it comes to SQL.

Answer 1

select loginFirstName, loginLastName, loginEmailAddress 
from login
where loginID not in
    (select distinct OrderuserID from orders)

You can also do it with a left join:

select loginFirstName, loginLastName, loginEmailAddress 
from login left join orders on loginID = OrderuserID 
where OrderuserID is null

Not sure which will execute faster; give it a try. The first is easier to understand, IMHO.

EDIT: “select distinct” means “return me the set of unique values of the field”. So, the subquery in the first SQL returns the set of users (their IDs) who do have orders. If a user has multiple orders, DISTINCT makes sure her ID is returned only once.