Kysely Query Builder — Vocabulary & Concepts Quiz

Kysely's type safety means that if you reference a column that doesn't exist in your schema definition, TypeScript shows a type error before you run the code. Table names, column names, join conditions, and return types are all checked at compile time — catching SQL errors that raw template literals would only reveal at runtime.

Kysely requires you to provide a Database interface: interface Database { users: UsersTable; posts: PostsTable; } where each table maps to an interface defining its columns and types. You pass this to new Kysely<Database>({ dialect }) and all queries are then type-checked against it.

The CamelCasePlugin handles the common mismatch between JavaScript naming (camelCase) and PostgreSQL column naming (snake_case). With it enabled, you write { firstName: 'John' } in TypeScript and Kysely maps it to the first_name column in SQL automatically — in both read and write directions.

Kysely's Migrator runs migration files that export up and down functions. Each function receives a Kysely instance (via db.schema) to create/alter/drop tables. A lock-protected migrations table tracks which migrations have run, ensuring they execute once and in order across environments.

Kysely plugins implement the KyselyPlugin interface with transformQuery and transformResult hooks. They intercept the compiled query node tree or raw results, enabling cross-cutting concerns like the CamelCasePlugin (transforming column names), soft-delete plugins (adding WHERE deleted_at IS NULL automatically), or custom logging.