Some Basic SQL interview questions for .net developers

• what is the difference between a union and a join?
• if you have a simple table of people with amounts of money - how would you get the name of the 2nd highest owner?
• how would you get the 2nd highest amount?
• if you have a table with name company amount and you you want to have sums per company how would you filter companies with 0 and null amounts ? 
• what is the difference between where and having?
• what is the difference between exists and a join?
• How would you create a database that needed to store a user name, password, and 2 phone numbers?
• What are some of the ways you can ensure unique data in a table?
• What is a CTE (common table expression) and when can it be used?
•  What would be the first few things you would check to try to improve a SQL call that is taking a long time to run?