[johnnyhillbilly]

SQL is the best when it comes to reasoning about information as tables. If you want to reason about tables as graphs, SQL becomes impractical for many applications.

Performance characteristics of different RDBMSs varies by orders of magnitude dependent on workloads.

Keys are also a flexible concept. They can be formalized to ensure referential integrity (domain), or you can join on whatever you want.

Recursive queries can handle some reasoning on graphs, but they quickly become impractical.

Even so: The flexibility SQL offers when you want to change which data you think should be related is tremendous - on HPC database machines :)