SQL - UPSERT LOOP through SELECT's resultset - tsql

Question

I have the following query that is returning two columns of data UserID & ServiceID from tables a & b
SELECT a.UserID, b.ServiceID FROM TableA a
JOIN TableB b
ON b.EventID = a.EventID
WHERE a.EventID = 1892286
AND a.Attendance IN (1,2)
The above query returns the following data set:
UserID ServiceID
1 33
2 44
3 55
I need to UPSERT into table c the following way:
If UserID & ServiceID exist in table c then take current value of column Scheduled from table c and add 1
If UserID & ServiceID do NOT exist in table c then insert record into table c with selected UserID and ServiceID from the above select statement and add 1 into the Scheduled column.
My question is how to do this for every single row that is returned by my select statement to ensure all results are UPSERTED into table c
I'm looking through looping case or if-else statements as possible solutions but any input is appreciated.
My attempt at the above:
IF EXISTS (SELECT sa.UserID as S_UserID, ce.ServiceID as S_ServiceID FROM TableA a
JOIN TableB b
ON b.EventID = a.EventID
WHERE a.EventID = 1892286
AND a.Attendance IN (1,2))
UPDATE TableC c
SET
UserID = S_UserID
ServiceID = S_ServiceID
Scheduled = Scheduled + 1
WHERE UserID = S_UserID
AND ServiceID = S_ServiceID
ELSE
INSERT INTO TableC
(UserID,ServiceID,Scheduled)
VALUES
(S_UserID,S_ServiceID,1)
END

Answer 1

If you are on newer version SQL Server (this question is tagged with sql-server-2016), then you can use the MERGE statement.
Here is a somewhat simplified example based on your post:
--create test tables
CREATE TABLE #a (UserID int);
CREATE TABLE #b (UserID int, ServiceID int);
CREATE TABLE #c (UserID int, ServiceID int, Scheduled int);
--insert some data matching the question
INSERT INTO #a
VALUES(1), (2), (3), (4), (5);
INSERT INTO #b
VALUES (1, 33), (2, 44), (3, 55)
INSERT INTO #c
VALUES(2, 44, 3), (4, 66, 1)
--ensure we have data
SELECT * FROM #a
SELECT * FROM #b
SELECT * FROM #c
--use the MERGE statement
MERGE #c AS tgt
USING(SELECT a.UserID, b.ServiceID FROM #a a JOIN #b b ON a.UserID = b.UserID) src
ON (tgt.UserID = src.UserID AND tgt.ServiceID = src.ServiceID)
WHEN NOT MATCHED THEN
INSERT (UserID, ServiceID, Scheduled)
VALUES(src.UserID, src.ServiceID, 1)
WHEN MATCHED THEN
UPDATE
SET Scheduled += 1;
--check what's been inserted/updated
SELECT * FROM #c
Be aware that MERGE sometimes gave performance implications, so use it with caution. More info about MERGE here.
Hope this helps!

Answer 2

One way to do this is by executing different statements for inserting and updating.
-- INSERT INTO Temporary Table
SELECT sa.UserID,
ce.ServiceID
INTO #tempC
FROM TableA a
JOIN TableB b ON b.EventID = a.EventID
WHERE a.EventID = 1892286
AND a.Attendance IN (1,2)
-- UPDATE Existing ROWS
UPDATE c
SET c.Scheduled = c.Scheduled + 1
FROM #tempC c
INNER JOIN TableC d
ON c.UserID = d.UserID
AND c.ServiceID = d.ServiceID
-- INSERT new rows
INSERT INTO TableC (UserID, ServiceID,Scheduled)
SELECT c.UserID, c.ServiceID, 1
FROM #tempC c
LEFT JOIN TableC d
ON c.UserID = d.UserID
AND c.ServiceID = d.ServiceID
WHERE d.UserID IS NULL
-- DROP temporary table
DROP TABLE #tempC

Answer 3

A loop is not efficient
ca and ce are not valid alias so I had to guess the valid alias
Start with a backup in case you do it wrong.
update c
set c.serviceID = c.serviceID + 1
FROM TableA a
JOIN TableB b
ON b.EventID = a.EventID
and a.EventID = 1892286
AND a.Attendance IN (1,2)
join TableC c
on c.UserID = a.UserID
and c.ServiceID = b.ServiceID;
INSERT INTO TableC (UserID, ServiceID, Scheduled)
SELECT a.UserID, b.ServiceID, 1
FROM TableA a
JOIN TableB b
ON b.EventID = a.EventID
and a.EventID = 1892286
AND a.Attendance IN (1,2)
where not exists (select 1
from TableC c
where c.UserID = a.UserID
and c.ServiceID = b.ServiceID
);