union along with a where clause

Question

I want to do something like this, but I know that this would lead to error

select * from t1 a
union 
select * from t2 b
union 
select * from t3 c
where b.col1!=c.col1

What should I be using in order to achieve above?..

Thanks !!

Answer 1

I think that you want NOT EXISTS:

select * from t1
union 
select * from t2
union 
select * from t3
where not exists (
  select 1 from t2
  where t2.col1 = t3.col1
)

With this query t3's rows will be included in the final result only if the value of col1 does not exist also in any of t2's rows.