This is an ugly hack to evaluate if relational calculus (specifically, Domain Relational Calculus) can be better suited than SQL for most simple queries with a few joins.
Two command-line arguments are expected. The first gives the query variables in output order. The second gives a query in DRC using a superset of the query variables and possibly additional string constants.
Example query on the example.txt database:
$ ./relq S,SD,L,LD 'student(S,SD) && immatriculated(S,"2016") && lecture(L,LD) && registered(S,L)' < example.txt
jane "Jane Dane" algebra1 "Algebra 1"
jane "Jane Dane" proglang1 "Introduction to Programming Languages"
john "John Doe" algebra1 "Algebra 1"
Try also the --debug switch to see the SQL produced internally.
If some column is not interesting an asterisk can be used instead of a variable name.
$ ./relq S,SD 'student(S,SD) && registered(S,*)' < example.txt
jack "Jack of all Trades"
jane "Jane Dane"
john "John Doe"
This queries all students that are registered for any lecture at all.
Negation is done with an exclamation mark:
$ ./relq S 'student(S,*) && !registered(S,"proglang1")' < example.txt
john
External variable bindings can be given with a -vVARIABLE=VALUE argument:
$ L=proglang1
$ ./relq -vL="$L" S 'student(S,*) && !registered(S,L)' < example.txt
john
This is to free the caller from the burden of escaping the value correctly.
Important features are still missing, like integer values, schema syntax and comparison predicates.