The Object Primer is supposedly (one of) the best book(s) to understand OO and UML
Page 12: Most business applications store data in some sort of database, often a relational one, yet the UML does not include a standard way to model data schemas. The implication is that as long as you are building business software that does not include a suser interface, or a database, then the UML might be sufficient for your needs. Personally, I have never built a system that was so narrowly constrained.”
Thus being the case, why a subtitle “Agile Model-Driven Development with UML 2.0″