数据库语句问题

Mysql中表student_table(id,name,birth,sex)，查询张姓、李姓的学生总人数，错误的是（）？

 

A

select sum(case when name like '张%' then 1 else 0 end) as zhang_first_name ,
sum(case when name like '李%' then 1 else 0 end) as li_first_name
from
student_table;

B

select count(case when name like '张%' then 2 else null end) as zhang_first_name ,
count(case when name like '李%' then 2 else null end) as li_first_name
from
student_table;

C

select count(case when name like '张%' then 1 else 0 end) as zhang_first_name ,
count(case when name like '李%' then 2 else 0 end) as li_first_name
from
student_table;

D

select sum(case when name like '张%' then 1 else null end) as zhang_first_name ,
sum(case when name like '李%' then 1 else null end) as li_first_name
from
student_table;

错误的是c

因为AD均为sum函数，返回1则加一，返回0和不变，返回null和也不变，

B和C都是使用的count，返回无论是0，1，2都会加一，但是饭返回null不会加一，则B正确C是错的