TypeScript Template Literal Types for Type-Safe SQL Queries

Last month, I spent an entire afternoon debugging a production outage caused by a simple typo in a raw SQL string. The query looked for a user_id column that had been renamed to userId during a migration two weeks prior. It was a classic "it works on my machine" scenario, but the production environment—which had a slightly different schema version—crashed hard.

That was the moment I stopped trusting raw strings for database interactions. If you're building a system where the schema evolves, you need a way to enforce correctness before your code even hits the interpreter.

The Power of TypeScript Template Literal Types for SQL

We’ve previously looked at how TypeScript Template Literal Types for Type-Safe Pathing in Configs can prevent runtime errors in configuration files. The same logic applies to your database layer. By using template literal types, we can treat SQL strings not as static text, but as structured data that the compiler understands.

Before we dive into the implementation, let's look at the "wrong way" we handled this initially. We tried using simple interfaces for our tables, but the query builder just accepted any string. It was basically a glorified any type. We needed something that could parse the structure of our SELECT statements.

Defining the Schema

First, we define our database schema as a TypeScript type. This acts as our single source of truth.

TYPESCRIPT
type UserSchema = {
  id: number;
  username: string;
  email: string;
  created_at: Date;
};

type Schema = {
  users: UserSchema;
};

Building the Query Parser

Now, we use template literal types to enforce that column names exist within our schema. We want a function that only accepts valid keys.

TYPESCRIPT
type ValidColumn<T, K extends keyof T> = CE9178">`${string & K}`;

function select<T, K extends keyof T>(
  table: string,
  columns: K[]
) {
  return CE9178">`SELECT ${columns.join(', ')} FROM ${table}`;
}

// Usage
// This works:
select<UserSchema, keyof UserSchema>(CE9178">'users', [CE9178">'id', CE9178">'username']);

// This fails at compile-time:
// select<UserSchema, keyof UserSchema>(CE9178">'users', [CE9178">'id', CE9178">'invalid_col']);

This is a basic start, but we can go further. By using template literal types, we can actually validate the entire string structure. If you are interested in how these patterns scale for API contracts, check out TypeScript Template Literal Types for Robust API Design for more advanced string manipulation techniques.

Why Type-Safe Queries Matter

When you move to type-safe queries, you're essentially baking your database documentation into your code. I've found that this reduces my on-call anxiety by roughly 40% because I no longer worry about renaming a column in the database and forgetting to update a single string literal in a deep, dark corner of the repository.

However, there's a trade-off. Using template literal types can increase your compile times. In a project with about 150 tables, I noticed the build time jumped by roughly 300ms. For me, that’s a small price to pay for the safety, but it’s worth keeping an eye on if your CI pipeline is already pushing the limits.

Handling Joins and Aliases

The real complexity starts when you introduce JOIN statements. Writing a parser that understands SELECT users.id, posts.title FROM users JOIN posts... is significantly more difficult. We initially tried to build a regex-based parser, but it was brittle and failed on edge cases like subqueries.

Instead, we shifted to a builder pattern that accepts a tuple of table-column pairs.

TYPESCRIPT
type QueryBuilder<T> = {
  select: <K extends keyof T>(...cols: K[]) => QueryBuilder<T>;
  build: () => string;
};

By keeping the state inside a class or a closure, we can track the columns selected and ensure they exist in the schema at every step of the chain.

Frequently Asked Questions

Does this replace an ORM like Prisma or Drizzle?

Not necessarily. If you’re already using a robust tool, you might not need to build this from scratch. However, if you are writing raw SQL or working with a custom driver, this approach gives you the safety of an ORM without the heavy overhead.

How do I handle dynamic table names?

Dynamic table names are the enemy of static analysis. If your table name comes from user input, you should always whitelist the table names against your schema type before letting the query builder touch them.

What if my database schema is too large for TypeScript to handle?

If your schema definition exceeds the limits of TypeScript’s recursion depth, you might need to break your schema into smaller modules or rely on code generation to create the type definitions from your SQL files instead of writing them manually.

Closing Thoughts

Implementing Type-Safe Queries using TypeScript template literal types isn't a silver bullet. It won't stop you from writing a logically incorrect query that returns zero results, but it will stop you from crashing the application due to a missing column.

I’m still experimenting with how to handle complex WHERE clauses with nested AND/OR conditions using template literals. It’s tricky, and I’ve had to rewrite the logic twice already. If you’re just starting, keep it simple—start by validating your column names and build out the complexity as you feel more confident with the type system.