Question

I’m trying to get a query to compare two tables by LEFT OUTER JOINing the two and finding any results that don’t have a match in the right table. Problem is I have a composite key of TicketNo, ItemNo and UniqueID, and I need to only compare rows that have the highest UniqueID for any given TicketNo.

I need the data so my program knows whether it needs to update a Working copy table and what rows it needs to update if so.

Basically what I want is this:

SELECT TicketNo
FROM History t1
LEFT OUTER JOIN Working t2 on 
max(t1.[UniqueID])=t2.[UniqueID] 
AND t1.[TicketNo]=t2.[TicketNo]
AND t1.[ItemNo]=t2.[ItemNo]
WHERE t2.[TicketNo] IS NULL 

But I can’t use the aggregate function max here. I don’t know how I can use a subquery (or failing that a CTE, but I would prefer a subquery) to get only the max unique ID for a given TicketNo. I can’t only join on the highest uniqueID in the table.

My data looks like this, assuming two duplicates:

TicketNo    UniqueID    ItemNo
15         1270662207   1
15         1184857061   1

In this case I only want the first row to return. I don’t care if my Working table has the old rows with TicketNo 15 or not.

Answer 1

DECLARE @History TABLE(UniqueID INT, TicketNo INT, ItemNo INT);
DECLARE @Working TABLE(UniqueID INT, TicketNo INT, ItemNo INT);

INSERT @History SELECT 15,1270662207,1
      UNION ALL SELECT 15,1184857061,1;

INSERT @Working SELECT 15,1184857061,1;

SELECT h.TicketNo, h.UniqueID
FROM 
(
    SELECT TicketNo, ItemNo, MAX(UniqueID) AS UniqueID
    FROM @History GROUP BY TicketNo, ItemNo
) AS h
LEFT OUTER JOIN 
@Working AS w
    ON h.TicketNo = w.TicketNo
    AND h.ItemNo = w.ItemNo
    AND h.UniqueID = w.UniqueID
WHERE w.TicketNo IS NULL;

Results:

TicketNo    UniqueID
----------- -----------
1270662207  15