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

Since by string comparison 'Offer' < 'Order' < 'Received', you can use the classic way with a group by subquery, to find the "latest" row:

select 
  p.Campaign,
  p.Event_Type,
  case p.Event_Type
    when 'Order' then p.Quantity_Order
    when 'Offer' then p.Quantity_Offer
    when 'Received' then p.Quantity_Received
  end as Quantity
from (
  select Campaign, max(Event_Type) as Event_Type
  from Purchasing
  group by Campaign
) mx
natural join Purchasing p
order by p.Campaign

Result:

| Campaign | Event_Type | Quantity |
| -------- | ---------- | -------- |
| C001     | Received   | 310      |
| C002     | Received   | 190      |
| C003     | Order      | 450      |
| C004     | Order      | 630      |
| C005     | Offer      | 400      |
| C006     | Offer      | 300      |

View on DB Fiddle

But if you were not so lucky with your Event_Type values, or if you want a more solid (more reliable) solution, you would need to define a custom order with either FIELD(), FIND_IN_SET(), a case statement, or a sort mapping (derived) table. Here a way using FIELD():

select 
  p.Campaign,
  p.Event_Type,
  case p.Event_Type
    when 'Order' then p.Quantity_Order
    when 'Offer' then p.Quantity_Offer
    when 'Received' then p.Quantity_Received
  end as Quantity
from Purchasing p
where p.Event_Type = (
  select p1.Event_Type
  from Purchasing p1
  where p1.Campaign = p.Campaign
  order by field(p1.Event_Type, 'Offer', 'Order', 'Received') desc
  limit 1
)
order by p.Campaign

View on DB Fiddle