When SQL Server 2008 R2 was released several new Reporting Services expressions were made available that would help join multiple datasets together.  Those functions were the Lookup Function, LookupSet Function and the MultiLookup Function.  My general recommendation for these function is not to use them when you have two dataset from the same data source.  They’re really intended for connecting datasets together that are not directly related because otherwise I could just handle it in my source query.

My experience with them had been fairly academic until recently when I had a real scenario where the Lookup Function would be beneficial.  I was working with a customer that had a report requirement that needed data from DB2 and Oracle in a single tabular report.  What made this circumstance more difficult was that the join between the DB2 dataset and the Oracle dataset was not a single column but two. 

Seems basic enough right?  If we were writing T-SQL we would simply do a join on multiple columns with an AND between them.  Because these are two different data sources in Reporting Services I had to get a little more creative.  If you’ve used the Lookup function in SSRS (template below) expressions then you know it can only accept one field from the source and one from the destination.  So what if you have two columns to join on?

Lookup(source_expression, destination_expression, result_expression, dataset)

The approach I took was to concatenate the two join columns together and then place the concatenated value in the Lookup function.  I also placed a literal string between the two columns to protect myself from coincidental matches.  To make the solution a little cleaner I created a calculated field on each dataset that handled the expression.  So both datasets had a calculated field I called JoinColumn and was defined similar to this:

=Fields!JoinColumn1.Value + ”||” + Fields!JoinColumn2.Value

Notice the double pipes that are used here to protect from coincidental matches.  With these calculated field created on each dataset now my Lookup function would be fairly standard.  Here’s my end result:

Lookup(Fields!JoinColumn.Value, Fields!JoinColumn.Value, Fields!ReturnColumn.Value, “DataSet2”)

I hope this helps as you find practical uses for the Lookup Function in Reporting Services.