[ocharles]

SQL as a string is not composable. I cannot give you a small query and have you arbitrarily join against it - I have to give you bits of strings and you have to glue them together in the right way. Expressing SQL as its AST lets you write combinators between ASTs, and thus get at least some composability. The stronger your type system, the better guarantees you can get that the composition is sound (closure).