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SqlType

> Links
> Description
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.
> Goals
  • Enable compile-time validation of SQL queries in TypeScript.
  • Infer query result shapes based on SELECT statements and schema inputs.
  • Support advanced SQL constructs like joins, aliases, and functions all in the type layer.
> 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;
  }
*/
> Tech Stack
  • TypeScript
> Learning Outcomes
  • Building domain-specific compilers using the TypeScript type system
  • Designing and modeling SQL ASTs using literal and mapped types
  • Creating safe APIs with compile-time validation
  • Understanding recursion limits and type inference depth in TS