Question

Query 1:

SELECT * 
FROM user_d1 
WHERE EXISTS (SELECT 1 
              FROM `user_d1` 
              WHERE birthdate BETWEEN '1989-08-04' AND '1991-08-04') 
ORDER BY timestamp_lastonline DESC 
LIMIT 20

Query 2:

SELECT * 
FROM user_d1 
WHERE birthdate BETWEEN '1989-08-04' AND '1991-08-04' 
ORDER BY timestamp_lastonline DESC 
LIMIT 20

And what I really don’t understand: why does Query 2 return the wrong results? It returns a list ordered first by birthdate and then by timestamp_lastonline…

Answer 1

Query 1 : If at least one record between the dates exists then the entire talbe is retrieved.

Query 2 : Only records between the dates are retrieved.

Read here for how EXISTS works.