I've got 2 tables Order & Despatch and I'm trying to add some keys to the tables. The OrderNo field is a primary key in Order and a foreign key in Despatch, but it won't let me add the key as there are obviously some records in Despatch that don't exist in Order. I now want to delete these rogue records. My first attempt was as follows but didn't work, any suggestions

delete from Despatch
(select b.ordno from Despatch b
full outer join Order a
on b.ordno = a.ordno
where a.ordno is null)

SELECT * FROM Despatch WHERE OrderNo NOT IN
(SELECT Ordno FROM Order)

This will show you all the rows in Despatch that don't have an OrdNo in Order.

Just so you can take a look and see if you really want to delete those rows.

Then you can run:

DELETE FROM Despatch WHERE OrderNo NOT IN
(SELECT Ordno FROM Order)

Many thanks, worked a treat.
I was obviously over complicating things