Are GUID collisions possible?

sql-server guid
Jason Baker picture Jason Baker · Oct 8, 2008 · Viewed 74.8k times · Source

I'm working on a database in SQL Server 2000 that uses a GUID for each user that uses the app it's tied to. Somehow, two users ended up with the same GUID. I know that microsoft uses an algorithm to generate a random GUID that has an extremely low chance of causing collisons, but is a collision still possible?

Answer

Tom Ritter picture Tom Ritter · Oct 8, 2008

Basically, no. I think someone went mucking with your database. Depending on the version GUID you're using the value is either unique (for things like version 1 GUIDs), or both unique and unpredictable (for things like version 4 GUIDs). SQL Server's implementation for their NEWID() function appears to use a 128-bit random number, so you're not going to get a collision.

For a 1% chance of collision, you'd need to generate about 2,600,000,000,000,000,000 GUIDs.

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