create table counttest(name char(1), age char(2));
insert into counttest(name, age)
values('a','14'),
('a','15'),
('a','15'),
('b',NULL),
('b','16'),
('c','17'),
('d',null),
('e','');
select name, count(1),count(*),count(age), count(distinct(age))
from counttest
group by name;
查询结果是