select count(distinct name) from student_table ;
select count(name) from (
select distinct name from student_table
) t1
select count(name) from (select name,count(*) as c1 from student_tablegroup by namehaving c1 > 1)t1
select count(name) from (
select name from student_table
group by name) t1;
select count(name) from ( select name, count(*) as c1 from student_table group by name having c1 = 1 ) t1;这样c,对于d 因为group by 具有去重的效果,所以对