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

SELECT x.campaign
     , COALESCE(z.event_type,y.event_type,x.event_type) event_type
     , COALESCE(z.quantity_received,y.quantity_order,x.quantity_offer) quantity
  FROM purchasing x
  LEFT 
  JOIN purchasing y 
    ON y.campaign = x.campaign 
   AND y.event_type = 'order'
  LEFT 
  JOIN purchasing z 
    ON z.campaign = y.campaign 
   AND z.event_type = 'received'
WHERE x.event_type = 'offer'

http://www.sqlfiddle.com/#!9/2b9394/37

Answer 2

Use this for your query.

this is based you first make your offer, then comes tghe order and at last comes the received, when 1 of them in that order is empty, then 1 prior must be the current state.

SELECT
Campaign,
IF(QReceived IS NOT NULL, "Received",
   IF(QOrder IS NOT NULL, "Order",
      IF(QOffer IS NOT NULL, "Offer", NULL))) Event_Type,
IF(QReceived IS NOT NULL, QReceived,
   IF(QOrder IS NOT NULL, QOrder,
      IF(QOffer IS NOT NULL, QOffer, NULL))) qty
      FROM
(SELECT 
 Campaign,
SUM(Quantity_Received) QReceived,

   SUM(Quantity_Order) QOrder,
   SUM(Quantity_Offer) QOffer

FROm Purchasing
GROUP BY Campaign) t1

Which gives you foolowing result

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

http://www.sqlfiddle.com/#!9/2b9394/34

Answer 3

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.

Answer 4

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