One of my favorite interview questions that tips even seasoned SQL guys (maybe because it's too simple) is around querying data that involves a self join.

Question:
Given an Employee table which has 3 fields - Id (Primary key), Salary and Manager Id, where manager id is the id of the employee that manages the current employee, find all employees that make more than their manager in terms of salary. Bonus: Write the table creation script.

Read more about the solution at SQL self join interview question on my programming interviews blog's post (http://www.programminginterviews.info/2013/03/sql-self-join-or-sub-query-interview-question-employee-manager-salary.html).