Mysql中表student_table(id,name,birth,sex), 查询不重复的姓名总数, 错误的是( )？
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_table group by name having c1 > 1)t1
select count(name) from (select name from student_table group by name) t1;
