Question

I have a restaurants. So I have a “menu” table and an “orders” table. I want to find when a given dish was most recently ordered. So I know I have to use group by dishId at some point, but everything I try returns an error. The following query works, but it returns duplicates. I only want the most recent instance for each dish. Note that my “orders” table contains customer information that I am not presently interested in.

select o.timestamp, m.* 
from orders o 
inner join menu m 
on (o.dishId = m.dishId); 

Answer 1

I think the problem your having here is you need to include each of the columns you want to group by

select m.dishId, MAX(o.timestamp)
    from orders o 
    inner join menu m 
    on (o.dishId = m.dishId)
group by m.dishId; 

If you want to group by more columns you have to add them in both places. Example:

select m.dishId, m.dishName, MAX(o.timestamp)
    from orders o 
    inner join menu m 
    on (o.dishId = m.dishId)
group by m.dishId, m.dishName;