dbquery

dbquery is a small OCaml library + CLI that parses and evaluates a DSL for CSV-backed tables.

What it does

• Parses a query language with assignments and table expressions.
• Evaluates programs left-to-right with mutable variable bindings.
• Supports load, project, join, rename, print, and save.
• Validates CSV/runtime invariants with explicit RuntimeError messages.
• Uses a hash-indexed join implementation for faster key joins.

Requirements

• OCaml
• Dune (>= 3.16)
• csv library
• menhir

Build and run

dune build

Run a sample program:

dune exec dbquery -- data/test2.dbq

Run tests:

dune test

Language quick reference

Program shape:

<command>; <command>; ...

Commands:

• Assignment: x := <table_expr>;
• Print: print <table_expr>;
• Save: save <table_expr> "path/to/file.csv";

Table expressions:

• Variable: x
• Load CSV: load "data/chem_grades.csv"
• Project columns: project ["ID"; "hw2"; "hw1"] from x
• Join on key: join a with b on "ID"
• Rename column: rename "old" to "new" from x
• Parenthesized expression: (<table_expr>)

Example

a := load "data/chem_grades.csv";
b := project ["ID"; "hw2"; "hw1"] from a;
print b;

Run it:

dune exec dbquery -- ./example.dbq

Runtime behavior and guarantees

• Loaded CSVs must be non-empty.
• Loaded CSVs must be rectangular.
• Loaded CSV headers must be unique.
• project requires requested columns to exist and be unique in the list.
• join is an inner join on equality and emits the key column once.
• join preserves deterministic row order and indexes the right table by key.
• rename fails if the target column name already exists.

Project layout

• lib/: AST, lexer/parser frontend, evaluator.
• bin/main.ml: CLI entrypoint.
• test/: deterministic parser/runtime/integration tests.
• data/: sample CSV inputs and .dbq programs.

Notes

Originally written for coursework and then cleaned up with stronger tests, clearer invariants, and evaluator optimizations.