Welcome to the Treehouse Community

The Treehouse Community is a meeting place for developers, designers, and programmers of all backgrounds and skill levels to get support. Collaborate here on code errors or bugs that you need feedback on, or asking for an extra set of eyes on your latest project. Join thousands of Treehouse students and alumni in the community today. (Note: Only Treehouse students can comment or ask questions, but non-students are welcome to browse our conversations.)

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and a supportive community. Start your free trial today.

Development Tools Database Foundations Joining Relational Data Between Tables in SQL Keys and Auto-Incrementing Values

Elena Paraschiv
Elena Paraschiv
9,938 Points

Unique key_ can or cannot be NULL?

In the video Andrew says in the beginning that unique keys can be null , unless otherwise specified. And then later when he gives the example he says that they cannot be null. Imgur


So how is it like?

1 Answer

Roy Penrod
Roy Penrod
19,809 Points

Only one row in the database may have a null unique key. The reason is the unique key is used to ensure that no two rows have the same data in them.

Here's an answer from Stack Overflow that could help clear up the difference between a primary key and a unique key.