I’ve not spoke of the Caché database previously; this is a product from InterSystems that uses MUMPS (now called M) as its database engine. The M language and database are thoroughly intertwined; database accesses and so on are just variable assignments, etc.
MUMPS goes back to the 1960s, to the days of COBOL, LISP, and FORTRAN. However, in spite of its age, it is robust and thriving (quietly) today – especially in the finance and health care industries where Caché is widely used.
They have an excellent set of documentation available, including an online set.
On another note, it turns out that InterSystems is now being sued by a patent troll, and they’ve asked the Groklaw community to help find prior art that would invalidate the patent.