Join two tables on multiple columns with OR and label - sql

Question

I have two tables as shown here:
orderID
customerID
1
1001
2
1002
3
1003
4
1003
and the other one is like:
userID
Service1FirstOrderID
Serice2FirstOrderID
Service3FirstOrderID
1001
null
1
null
1002
2
null
null
1003
3
null
4
Now I want to join these two tables so that I can get every customer id with ServiceID that have been purchased.
UserID
Service
1001
2
1002
1
1003
1
1003
3
Any help would be appreciated.

Answer 1

It's possible to join on an IN
SELECT
so.userID
, CASE
WHEN o.OrderID = so.Service1FirstOrderID THEN 1
WHEN o.OrderID = so.Service2FirstOrderID THEN 2
WHEN o.OrderID = so.Service3FirstOrderID THEN 3
END AS Service
FROM Orders o
INNER JOIN ServiceOrders so
ON so.userID = o.customerID
AND o.OrderID IN (so.Service1FirstOrderID, so.Service2FirstOrderID, so.Service3FirstOrderID)
ORDER BY o.customerID;
userID
Service
1001
2
1002
1
1003
1
1003
3
Demo on db<>fiddle here

Answer 2

You have a heavily denormalized table structure, but it appears that this is not a join at all.
It seems to be purely a conditional unpivot of the second table, which you can do with CROSS APPLY
SELECT
t2.UserId,
v.*
FROM table2 t2
CROSS APPLY (
SELECT 1
WHERE Service1FirstOrderID IS NOT NULL
UNION ALL
SELECT 2
WHERE Service2FirstOrderID IS NOT NULL
UNION ALL
SELECT 3
WHERE Service3FirstOrderID IS NOT NULL
) v(Service);
db<>fiddle

Answer 3

You can achieve this using CTE.
;WITH CTE_ServiceOrder as
(
SELECT UserId, 1 AS ServiceId, Service1FirstOrderID as orderId
from ServiceOrders
where Service1FirstOrderId is not null
union all
SELECT UserId, 2, Service2FirstOrderID as orderId
from ServiceOrders
where Service2FirstOrderId is not null
union all
SELECT UserId, 3, Service3FirstOrderID as orderId
from ServiceOrders
where Service3FirstOrderId is not null
)
SELECT o.customerID, s.ServiceId FROM CTE_ServiceOrder as s
INNER JOIN Orders as o
on o.orderID = s.orderid
order by o.customerID
Thanks to #Lukstorms for create script.
You can refer to the dbfiddle