Random Value Per Row in SQL

Occasionally, you need to update a table with a random value per row. And thanks to some optimizations SQL Server does, it’s not exactly straight forward.

If you just try to update with a random value like this, every row ends up with the same ‘random’ value.

update MyTable
   set SomeValue = rand()

This is because SQL Server only runs the rand() once because it doesn’t depend on any value from the row. My next thought was to see the rand() with a value from each row.

update MyTable
   set SomeValue = rand(Id) --Where Id is in Int

This wasn’t as random as I had hoped. Since my Id column was an identity column the ‘random’ numbers were almost sequential as well. For example, I got the following ‘random’ numbers for the following Id values:

So, I needed to come up with a way to get the seed value to vary for each row. So I decided to get the MD5 hash of the Id column.

update MyTable
   set SomeValue = rand(HASHBYTES('md5', convert(varchar, Id)))

That results in these values:

And that was random enough for my needs.