Is it possible to select from an alias in mysql?

Question

I created a table alias named tbl, and I want to select from that. But I am unable to do this. I know that my code is not correct nor optimized, but I am simply tes

Answer 1

try this out

select
     case
         when exists (select username from tbl where  username = 'bob' and  password = 'pass') then 'Username Exists'
     else 'Username does not exist'
     end as existanse_column
from  tbl
limit 1

DEMO HERE