isnull()select * from users where email = xxxx and isnull(deletedAt)
is nullselect * from users where email = xxxx and deletedAt is null
is not nullselect * from users where email = xxxx and deletedAt is not null
!isnull()select * from users where email = xxxx and !isnull(deletedAt)
select * from users where email = xxxx and not isnull(deletedAt)
isfull
当查询条件为 null,用指定字符替代select name, isfull(gender,未知) as gender from users where email = xxxx