Select from table if record found in another table

Question

I need to select some rows from Table 1 lets say if a value is found in Table 2. So I want to check if the value (I will enter the value from command line) is found in Table

Answer 1

This query will select from Table1 if :id exists in Table3, and from Table2 otherwise:

select  *
from    Table1
where   exists
        (
        select  *
        from    Table3
        where   id = :id
        )
union all
select  *
from    Table2
where   not exists
        (
        select  *
        from    Table3
        where   id = :id
        )

Answer 2

You can do something like this:

-- If value is found in table2, select from table1
select * -- <- use padding if necessary 
  from table1
 where exists (select 1
                 from table2
                where myField = value)

union all

-- If value is not found in table2, select from another_Table
select * -- <- use padding if necessary
  from another_Table
 where not exists (select 1
                     from table2
                    where myField = value)