choosing columns on a condition using mysql

i have two tables in my db,

Table A
a_id,email,col2,col3

and

Table B
b_id,email

what i want to do is formulate a single query that can chooses the column names per boolean.

i mean i have a set of a_id's that i want to fetch data for from Table A,

for a particular a_id, if the email exists in Table B, fetch col2, else fetch col 3

i want to formulate a query开发者_Python百科 something like below, but in a efficient way :)

select 
if (email exists table B)  
col 2, 
else 
col 3 
as title
from Table A 
where a_ids in (1,2,3,4,5,6,7)

Try ths one -

SELECT a.a_id, IF(b_id IS NULL, col3, col2) FROM tablea a
  LEFT JOIN tableb b ON a.a_id = b.b_id
WHERE a.a_id IN (1,2,3,4,5,6,7);