More sql write tests: subquery, alias
Move handle to DbConnection; introduce JDBCHandle
Stub for sqlite3 db connections; WIP
Refine DbConnection method args a bit
Keep order of labels in Schema
unary -> isUnary
Provide labels of Schema
Move Label/Domain to RelationalAlgebra
Provide domains of Schema
Refine DbConnection interface
Move domain to own class file, add subtypes
Implement open, query, insert for sqlite3
Make Label a type alias for String
Implement execute for sqlite3
Document return types
Test sqlite3
Fix bug: separate placeholders