Pretty much any SQL book will cover it those, and there's a bunch of online SQL tutorials. UDF means user defined function, so if there's some function you want to perform in SQL but that function doesn't come out of the box, you can just write your own. And those can be defined in non-SQL syntax, such as UDF's written in python or C++, which can be pretty handy.
dmoy|8 days ago
Also sometimes Lua, which is kinda a nice middleground between c++ efficiency and python ease of writing