How to return rows from left table not found in right table?

Question

I have two tables with similar column names and I need to return records from the left table which are not found in the right table? I have a primary key(column) which will

Answer 1

This is an example from real life work, I was asked to supply a list of users that bought from our site in the last 6 months but not in the last 3 months.

For me, the most understandable way I can think of is like so:

--Users that bought from us 6 months ago and between 3 months ago.
DECLARE @6To3MonthsUsers table (UserID int,OrderDate datetime)
INSERT @6To3MonthsUsers
    select u.ID,opd.OrderDate
        from OrdersPaid opd
        inner join Orders o
        on opd.OrderID = o.ID
        inner join Users u
        on o.BuyerID = u.ID
        where 1=1 
        and opd.OrderDate BETWEEN DATEADD(m,-6,GETDATE()) and DATEADD(m,-3,GETDATE())

--Users that bought from us in the last 3 months
DECLARE @Last3MonthsUsers table (UserID int,OrderDate datetime)
INSERT @Last3MonthsUsers
    select u.ID,opd.OrderDate
        from OrdersPaid opd
        inner join Orders o
        on opd.OrderID = o.ID
        inner join Users u
        on o.BuyerID = u.ID
        where 1=1 
        and opd.OrderDate BETWEEN DATEADD(m,-3,GETDATE()) and GETDATE()

Now, with these 2 tables in my hands I need to get only the users from the table @6To3MonthsUsers that are not in @Last3MonthsUsers table.

There are 2 simple ways to achieve that:

1. Using Left Join:

   select distinct a.UserID
   from @6To3MonthsUsers a
   left join @Last3MonthsUsers b
   on a.UserID = b.UserID
   where b.UserID is null
   

2. Not in:

   select distinct a.UserID
   from @6To3MonthsUsers a
   where a.UserID not in (select b.UserID from @Last3MonthsUsers b)
   

Both ways will get me the same result, I personally prefer the second way because it's more readable.