MYSQL：错误1241（21000）：操作数应包含1列

Question

this should be working but I get an error somehow. I want to get the credits column from the table and multiply it by 100. The problem is to get the number of credits for a given student id and year and get the total payment. Assuming each credit is $100.

delimiter //
create function fTest (stuYear varchar(4), stuID varchar(4))
returns varchar(200)
begin
declare msg varchar(200) default '';
if (stuYear = '' or stuYear is null) then 
    select 'Please input a valid year' into msg;
elseif (stuID = '' or stuID is null) then 
    select 'Please input a student  id' into msg;    
else
begin

if (msg = '' or msg is null) then
    select ('No result found for student ID: ', stuID, ' at year: ', stuYear) into msg; 
select (credits * 100) into msg from Students_Courses natural join Courses where sid=stuID and year=stuYear group by credits;
return msg ;    
end if;    
end ;
end if;
end ;
//
delimiter ;

Answer 1

This is incorrect:

select ('No result found for student ID: ', stuID, ' at year: ', stuYear)

A select statement can contain multiple columns, but you shouldn't enclose them in parentheses. Also, that would return multiple values, which you can't select into a single message.

So, I guess you wanted to concat the values into a single value. You can do that using the concat function, like so:

select concat('No result found for student ID: ', stuID, ' at year: ', stuYear)

Btw, a normal assignment using the concat function should also work in a trigger:

SET msg = concat('No result found for student ID: ', stuID, ' at year: ', stuYear);

See MySQL concat function

PS: In the next statement, you also got parentheses: (credits * 100) In this case it accidentally will work, because it's a single expression. They don't have any functional value, though, and might as well be removed.