Thanks. So specifying A.A = 1 will return all matches for AD.A = 1 , but only distinct rows from the rest of A?

Yes - having played with test data I've now realised that. Thanks for the reply.

However, I came across another strange thing. Using test data:

Table A

ID

1

2

3 

4 

5 

Table AD

ID     ADID

1     1

1     2

2     3

2    ...

How do LEFT JOINS and WHERE clauses work when you include a CASE stsement in the SELECT clause?

e.g 

SELECT

  O.OrderNo,

  CASE WHEN A.TopUpLoan = 1 THEN A.VehAmt ELSE 0 END As...