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Multi-column keys

Two new datasets are available in your R session:

subjects: contains the subject each student is taking each semester this year
locations: contains the room allocations for each subject

Your goal is to build a new data.table with the locations of each subject for each student.

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Right join locations to subjects using the "subject" and "semester" columns in both data.tables as two separate join key columns. You should get an error.

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