One of the first thing I always say to the new hires that are going to learn Maximo is how much important is to have a good knowledge of SQL language. It is really important to build application queries, Birt reports, database relationships, complex table domains, conditional expressions and many other Maximo configurations.
The important thing to understand is that SQL is more declarative than procedural like C or Java so it may require a mind shift for common developers. It is hard to explain in a short post like how to develop a good SQL query but well written SQL code can solve many Maximo performances issues.
Here is a good SQL tutorial that you can download. It is for IBM DB2 but it is 95% valid for any RDBMS and it is really well written and concise.