YZ
Size: a a a
D
D
D
YZ
D
D
YZ
A
OK
GS
YZ
GS
OK
GS
YZ
OK
GS
select count(*)
from states s
where exists(
select id from cities c
where c.state_id = state.id
)OK
select count(*)
from states s
where exists(
select id from cities c
where c.state_id = state.id
)YZ