SQL case return more than 1 row write a string

I have a subquery in SELECT and sometimes it returns more than one row. I want to solve the problem like this:

• when more than 1 row write a string like ‘multi’
• otherwise use the value description from comment table

Query:

select 
    s.id, 
    (select description from comment c where c.id = s.id) 
from student s;

thanks.

Answer

You can join, aggregate, and use a conditional expression:

select 
    s.id,
    case when min(c.description) <> max(c.description) 
        then 'multi' 
        else min(c.description) 
    end description
from student s
left join comment c on c.id = s.id
group by s.id

You can also do this with a subquery, which avoids outer aggregation (it is handy if you need more columns from students):

select
    s.*,
    (
        select 
            case when min(c.description) <> max(c.description) 
                then 'multi' 
                else min(c.description) 
            end
        from comment c
        where c.id = s.id
    ) description
from student s