SqlType

infers the result type of a SQL query at compile time, using only TypeScript's type system.

SqlType is a static SQL analysis and inference engine implemented entirely with TypeScript’s type system. it parses SQL-like query strings at compile time and infers the resulting data shape based on a provided schema. the system supports validation of selectors, function calls, joins, aliases, and SELECT * semantics - all within the type layer of TypeScript, with zero runtime logic. SqlType helps developers catch query issues during development, creating safer and more predictable data access layers in TypeScript-based systems.

features

compile-time SQL parsing

parses and analyzes SQL queries entirely at the type level using recursive conditional types.
full query shape inference

infers result structures from SELECT queries, including expressions and function calls.
support for joins and aliases

supports INNER, LEFT, RIGHT, and FULL OUTER JOINs with alias resolution and field mapping.
function usage validation

functions like COUNT(...) are checked at compile time for valid argument types and nesting.
zero-runtime

no runtime dependency; the entire system works within TypeScript’s type checker.

usage

// Example 1: Simple SELECT with alias
type Tables = {
  users: {
    id: number;
    name: string;
  };
};

type Query = "SELECT users.name AS username FROM users";

type Result = BuildQuery<Query, Tables>;

/*
  Result:
  {
    username: string;
  }
*/

// Example 2: COUNT aggregation with JOIN
type Tables = {
  users: {
    id: number;
    name: string;
  };
  posts: {
    id: number;
    user_id: number;
  };
};

type Query = "SELECT COUNT(posts.id) AS totalPosts FROM users LEFT JOIN posts ON users.id = posts.user_id";

type Result = BuildQuery<Query, Tables>;

/*
  Result:
  {
    totalPosts: number;
  }
*/

// Example 3: Multiple columns with alias and join
type Tables = {
  users: {
    id: number;
    name: string;
  };
  posts: {
    id: number;
    user_id: number;
    title: string;
  };
};

type Query = "SELECT u.name AS username, COUNT(p.id) AS total FROM users u LEFT JOIN posts p ON u.id = p.user_id";

type Result = BuildQuery<Query, Tables>;

/*
  Result:
  {
    username: string;
    total: number;
  }
*/

stack

TypeScript, TypeScript type-level programming