Difference between ROW_NUMBER() and RANK() in MSSQL

Contrary to popular belief, RANK() does not sort rows based upon how bad they smell. RANK() does much the same thing as ROW_NUMBER(), only it acknowledges ties in the columns specified in the ORDER BY clause, and assigns them the same rank. Where a tie occurs (as was the case for orders 6/3, and 1/5/, the numbers that would otherwise have been “used up” are skipped, and numbering resumes at the next available number. As you can see, RANK() leaves a gap whenever there is a tie.

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