I want to know to how many lists a user is assigned in average.
User ==== id name Lists_Users =========== user_id list_id List ==== id title
Is there a good way of receiving this number via SQL?
You only need the counts from the 2 tables and a division:
If you want more statistical information, you can write it using derived tables:
I have no idea which RDBMS you are using, but on SQL Server using CTEs (Common Table Expression) it might look like something similar:
There must also be dozen of other (better?) ways of doing this...