If you are looking for a fully relational database with a GUI front-end for Mac, I would suggest checking out 4th Dimension (4D) at www.4d.com
This database development platform has been around on the Mac since 1987. While you can do a lot of development without writing any code, you will soon want to dive in and start to learn to program in 4D. It is a PASCAL-like procedural language owing to it's original development in the 1980's.
It now supports the SQL language as well.
While the program is not for the faint of heart, I promise you will NOT outgrow it.
You will most likely be interested in the 4D Developer Standard.
It has everything you need to get started.
Hope that helps,