MySQL exists

 select  exists  (select  'A' where 5>10) as R;

+---+

| R |

+---+

| 0 |

+---+

Because     the evaluated query return   empty set, if return any value doesn't matter the value   it will return  1, doesn't matter if is null or 0.

select  'A' where 5>10;

Empty set (0.00 sec)

------------------------    exists   here will return 1

select  exists  (select  NULL) as R;

+------+

| R    |

+------+

|    1 |

+------+

1 row in set (0.00 sec)

-

 select  exists  (select  0) as R;

+---+

| R |

+---+

| 1 |

+---+

select 'Ambiorix' where exists(select 1);

+----------+

| Ambiorix |

+----------+

| Ambiorix |

+----------+