Three dimensional database table
We have all been there - consider the following example - first, the client says "every user shall only have one profile picture", so we add a field for that to the users table - half a year later, requirements change and a user actually needs to have n profile pictures.
Now, this seems only possible if you add a new table such as user_pictures to handle the new cardinality 1:n instead of 1:1. Oftentimes this can get very complicated. Whenever I come across this problem, I wonder why we don't use all three dimensions that we can think in. A two dimensional table is limited in a way that it is somewhat incomplete - what if, referring to our problem with the profile picture again, the picture field in the users table had a depth, and that depth made the field an array that perfectly represented both cardinalities 1:1 and 1:n at the same time.
Table fields would simply become arrays and automatically support both cardinalities - wouldn't that be something? At least I would use it. Is there something like it out there already?
Answer
Oracle has support for arrays as well as nested tables. Either seem to fit your requirements. These days though people prefer to model everything as tables and relationships to keep things simple and consistent and so modern RDBMSes don't generally support this stuff and I don't believe it ever made it into standard SQL either.