I like the FizzBuzz puzzle.

I use it to identify if an applicant

- prefers using a set or loop based method where both methods are possible

- knows enough T-SQL to generate a numbers list from scratch (or if the person knows a method to generate such a list based on a system table)

- understand how the CASE fuction works

- knows how deal with error messages (usually an applicant would simply add "ELSE number" leading to a conversion error)

This simple test is also a good start to talk about some rather basic SQL internals: (Why does SQL Server try to convert "fizz" to a numeric value?)

The test to find duplicate rows is a good test for solving a common task.