Case error of MySQL branch statement

In use MySQL（ The version is 8.0 The above ） case Error in branch statement , As long as it's used case will , The same is true for simple branch statements , Let's see , Here's the picture

This is one of the stored procedure scripts

create procedure proc_test9(in score int)
begin
case
when score > 90 and score <= 100 then select 'a';
when score > 80 then select 'b';
when score > 70 then select 'c';
else select 'd';
end case;
end;