How do you create a type-safe ORM query builder?

Use mapped types for table schemas, conditional types for query validation, and method chaining with type narrowing. Basic Schema Definition: type User = { id: number; name: string; email: string; age: number; }; type Post = { id: number; title: string; content: string; userId: number; }; Query Builder Implementation: type WhereClause = { [K in keyof T]?: T[K] | { gt: T[K] } | { lt: T[K] } | { in: T[K][] }; }; type SelectFields = (keyof T)[]; class QueryBuilder { constructor(private tableName: string) {} select(...fields: K[]): QueryBuilder { // Implementation return new QueryBuilder(this.tableName); } where(conditions: WhereClause): QueryBuilder { // Implementation return this; } execute(): Promise[]> { // Implementation return Promise.resolve([]); } } // Usage const userQuery = new QueryBuilder('users') .select('id', 'name', 'email') .where({ age: { gt: 18 } }) .execute(); // Returns Promise[]> Advanced Join Support: type JoinResult = T & U; class AdvancedQueryBuilder { constructor(private tableName: string) {} join( table: string, leftKey: K, rightKey: L ): AdvancedQueryBuilder> { // Implementation return new AdvancedQueryBuilder>(this.tableName); } select(...fields: K[]): AdvancedQueryBuilder> { // Implementation return new AdvancedQueryBuilder>(this.tableName); } } // Usage with joins const joinQuery = new AdvancedQueryBuilder('users') .join('posts', 'id', 'userId') .select('name', 'title'); // Type: Pick Benefits: • Compile-time validation of table schemas • Type-safe field selection • Prevents invalid join conditions • Excellent IDE autocomplete for database operations.