Query Language Construction

Communication between the developed system and the user takes place through a purpose-built, declarative query language. The language’s construction is based on the algebra presented in the previous chapter. Just as, for relational systems, relational algebra forms the basis for the SQL language — in our case, the developed algebra forms the basis for the RQL query language.

RQL stands for RetractorDB Query Language. Its syntax is very similar to SQL syntax. Bear in mind, however, that the correct term here is a False Friend. That is, it looks like SQL, but has little in common with it.

Valid statements in RQL currently begin with a handful of keywords. The most recognizable is the command starting with the keyword SELECT, followed by a list of attributes in the form of algebraic expressions — an algebra based on real numbers.

Statements are written in a text file. Its extension is conventionally .rql, but any other extension will also be accepted and processed. An RQL text file contains a sequence of statements beginning with defined keywords. Comments are prefixed with the # character.

The query language was implemented using the Antlr4 parser generator [5]. The RQL grammar is written down, defined, and after every modification is compiled into the language in which RetractorDB itself was built. Every statement in a query file that is not a comment is compiled, processed, and modifies the system’s internal state. A statement may span multiple lines — line continuation is signaled with a \ character at the end of the line.