Merge into with multiple updates and inserts

Question

Basically I have a SQL Server 2008 R2 database. The database has a table called Node and Link. Link contains a StartNodeId and EndNodeId relating to a Id in Node. The database also requires a Link table between Node and Link for quicker checking of say, is this Node related to this Link or which Nodes are related to this Link. The Link table contains a Identity key, NodeId and LinkId. My problem is when I am doing my inserts I am trying to use merge statements which do not seem to be able to do what I am trying

When I tried

MERGE INTO [RoadRoutingDatabase].[dbo].[NodeToLink] AS TARGET
USING (SELECT Id, StartNodeId, EndNodeId FROM [RoadRoutingDatabase].[dbo].[Link]) AS SOURCE
ON (TARGET.LinkId = SOURCE.Id)
WHEN MATCHED AND TARGET.NodeId = Source.StartNodeId THEN
    UPDATE SET TARGET.NodeId = SOURCE.StartNodeId,
               TARGET.LinkId = SOURCE.Id
WHEN MATCHED AND TARGET.NodeId = Source.EndNodeId THEN
    UPDATE SET TARGET.NodeId = SOURCE.EndNodeId,
               TARGET.LinkId = SOURCE.Id
WHEN NOT MATCHED BY TARGET AND TARGET.NodeId = Source.StartNodeId THEN
    INSERT (LinkId, NodeId)
    VALUES (SOURCE.Id, SOURCE.StartNodeId)
WHEN NOT MATCHED BY TARGET AND TARGET.NodeId = Source.EndNodeId THEN
    INSERT (LinkId, NodeId)
    VALUES (SOURCE.Id, SOURCE.EndNodeId)
WHEN NOT MATCHED BY SOURCE THEN
    DELETE;

I get the error message "An action of type 'WHEN MATCHED' cannot appear more than once in a 'UPDATE' clause of a MERGE statement"

If I try inserting Start Nodes and End Nodes seperatly e.g.

    --Insert Start Node To Link Relationships
    MERGE INTO [RoadRoutingDatabase].[dbo].[NodeToLink] AS TARGET
    USING (SELECT Id, StartNodeId FROM [RoadRoutingDatabase].[dbo].[Link]) AS SOURCE
    ON (TARGET.NodeId = SOURCE.StartNodeId AND TARGET.LinkId = SOURCE.Id)
    WHEN MATCHED THEN
        UPDATE SET TARGET.NodeId = SOURCE.StartNodeId,
                   TARGET.LinkId = SOURCE.Id
    WHEN NOT MATCHED BY TARGET THEN
        INSERT (LinkId, NodeId)
        VALUES (SOURCE.Id, SOURCE.StartNodeId)
    WHEN NOT MATCHED BY SOURCE THEN
        DELETE;

    --Insert End Node To Link Relationships
    MERGE INTO [RoadRoutingDatabase].[dbo].[NodeToLink] AS TARGET
    USING (SELECT Id, EndNodeId FROM [RoadRoutingDatabase].[dbo].[Link]) AS SOURCE
    ON (TARGET.NodeId = SOURCE.EndNodeId AND TARGET.LinkId = SOURCE.Id)
    WHEN MATCHED THEN
        UPDATE SET TARGET.NodeId = SOURCE.EndNodeId,
                   TARGET.LinkId = SOURCE.Id
    WHEN NOT MATCHED BY TARGET THEN
        INSERT (LinkId, NodeId)
        VALUES (SOURCE.Id, SOURCE.EndNodeId)
    WHEN NOT MATCHED BY SOURCE THEN
        DELETE;

I end up with links being deleted (not surprising) so basically I was wondering if anyone knew of a good way of doing this? If possible I would like to be able to do it still using a merge statement

Thanks

Edit: I have found a different way of merging this data using a different source, the problem is now solved.

Answer 1

Maybe I'm missing something but

The error message complains you can't have multiple WHEN MATCHED so you could convert

WHEN MATCHED AND TARGET.NodeId = Source.StartNodeId THEN
    UPDATE SET TARGET.NodeId = SOURCE.StartNodeId,
               TARGET.LinkId = SOURCE.Id
WHEN MATCHED AND TARGET.NodeId = Source.EndNodeId THEN
    UPDATE SET TARGET.NodeId = SOURCE.EndNodeId,
               TARGET.LinkId = SOURCE.Id

to

WHEN MATCHED AND TARGET.NodeId IN (Source.StartNodeId,Source.EndNodeId) THEN
    UPDATE SET TARGET.NodeId = CASE 
                                 WHEN TARGET.NodeId = Source.StartNodeId 
                                 THEN SOURCE.StartNodeId 
                                 ELSE Source.EndNodeId 
                               END,
               TARGET.LinkId = SOURCE.Id

But as the first branch of the CASE is hit when TARGET.NodeId = Source.StartNodeId and also sets TARGET.NodeId = Source.StartNodeId and similarly for the second branch then that seems to simplify to

WHEN MATCHED AND TARGET.NodeId IN (Source.StartNodeId,Source.EndNodeId) THEN
    UPDATE SET TARGET.LinkId = SOURCE.Id