MySQL 8.0

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CREATE TABLE Purchasing (
    Event_Type VARCHAR(255),
    Campaign VARCHAR(255),
    Quantity_Offer VARCHAR(255),
    Quantity_Order VARCHAR(255),
    Quantity_Received VARCHAR(255)
);

INSERT INTO Purchasing
(Event_Type, Campaign, Quantity_Offer, Quantity_Order, Quantity_Received)
VALUES 
("Offer", "C001", "300", NULL, NULL),
("Offer", "C002", "200", NULL, NULL),
("Offer", "C003", "500", NULL, NULL),
("Offer", "C004", "600", NULL, NULL),
("Offer", "C005", "400", NULL, NULL),
("Offer", "C006", "300", NULL, NULL),
("Order", "C001", NULL, "320", NULL),
("Order", "C002", NULL, "180", NULL),
("Order", "C003", NULL, "450", NULL),
("Order", "C004", NULL, "630", NULL),
("Received", "C001", NULL, NULL, "310"),
("Received", "C002", NULL, NULL, "190");
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


campaign
event_type
quantity


C001
Received
310

C002
Received
190

C003
Order
450

C004
Order
630

C005
Offer
400

C006
Offer
300


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(event_type, 'Received', 'Order', 'Offer')
                    limit 1
                   )
order by campaign;


campaign
event_type
quantity


C001
Received
310

C002
Received
190

C003
Order
450

C004
Order
630

C005
Offer
400

C006
Offer
300

campaign	event_type	quantity
C001	Received	310
C002	Received	190
C003	Order	450
C004	Order	630
C005	Offer	400
C006	Offer	300