Only display values of latest available event type

Question

SQL Fiddle:

CREATE TABLE Purchasing (
    Event_Type VARCHAR(255),
    Campaign VARCHAR(255),
    Quantity_Offer VARCHAR(255),
    Quantity_         


        

Answer 1

This is a prioritization query.

In MySQL 8+, use window functions:

select campaign, event_type,
       coalesce(Quantity_Offer, Quantity_Order, Quantity_Received) as quantity
from (select p.*,
             row_number() over (partition by campaign order by field(event_type, 'Received', 'Order', 'Offer')) as seqnum
      from Purchasing p
     ) p
where seqnum = 1;

In earlier versions, a simple method is a correlated subquery:

select campaign, event_type,
       coalesce(Quantity_Offer, Quantity_Order, Quantity_Received) as quantity
from Purchasing p
where event_type = (select p2.event_type
                    from Purchasing p2
                    where p2.campaign = p.campaign
                    order by field(p2.event_type, 'Received', 'Order', 'Offer')
                    limit 1
                   );

If you have a lot of data, you want an index on Purcahsing(campaign, event_type) for this query.

Here is a db<>fiddle.