A unique index in SQL
SQL interview exercise on Schema Design, difficulty 1 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Make it impossible for two customers to share an email address by adding a unique index on customers.email. Customers without an email are still fine: NULLs never count as equal, so a unique index allows many of them.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.