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Finding unique records from a subset of the database


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I know how to find unique records in my database using the "self join" business (actually I don't fully understand the logic, I'm just following the suggestions on the Help page!). Now I need to find records that are unique within a subset of the data. The records are for plant specimens and the relevant fields are "species" and "location." Like this:


Year Location Species Unique1 Unique2

2011 A 1 Y Y

2011 A 1 N N

2011 A 2 Y Y

2011 A 2 N N

2012 B 1 N Y

2012 B 1 N N

2012 B 2 N Y

2012 B 2 N N


"Unique1" is my existing variable: it tags as unique the first ever occurrence of a species. If next year I collect the same species at a different location it is not unique by that definition. What I need is new variable "Unique2" which tags as unique the first occurrent of a species in any location.


I have no idea how to do this and will be grateful of any help the forum can give me.

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