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

You have that error because there's no physical table involved in the query, because tbl is just the alias you created. If you want just to test if your username exists, execute this query:

SELECT CASE
WHEN id = 1 THEN 'Username Exists' ELSE 'Username does not exist'
END
FROM (SELECT 1 AS id, 'bob' AS username, 'pass' AS password) AS tbl

Answer 2

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