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Kysely’s primary goal is to provide end-to-end type safety for SQL queries. Every column reference, table name, and return type is validated at compile time.

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Database Type Parameter

Type safety starts with the database type parameter passed to Kysely<DB>: 
import { Kysely, Generated } from 'kysely'

interface Database {
  person: {
    id: Generated<number>
    first_name: string
    last_name: string | null
    age: number
  }
  pet: {
    id: Generated<number>
    name: string
    owner_id: number
    species: string
  }
}

const db = new Kysely<Database>({
  dialect: /* ... */
})
This single type parameter powers all type inference throughout Kysely.

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Column Selection Type Inference

Kysely infers the return type based on selected columns: 
// Type: { first_name: string; age: number }[]
const result = await db
  .selectFrom('person')
  .select(['first_name', 'age'])
  .execute()

result[0].first_name // ✓ string
result[0].age // ✓ number
result[0].last_name // ✗ Error: Property 'last_name' does not exist

​
Aliased Columns

// Type: { firstName: string; personAge: number }[]
const result = await db
  .selectFrom('person')
  .select([
    'first_name as firstName',
    'age as personAge'
  ])
  .execute()

result[0].firstName // ✓ string
result[0].personAge // ✓ number
result[0].first_name // ✗ Error: Property 'first_name' does not exist

​
SelectAll

// Type: { id: number; first_name: string; last_name: string | null; age: number }[]
const result = await db
  .selectFrom('person')
  .selectAll()
  .execute()

​
Table Reference Validation

Only tables defined in the database interface can be referenced: 
// ✓ Valid
await db.selectFrom('person').selectAll().execute()

// ✗ Error: Argument of type '"invalid_table"' is not assignable
await db.selectFrom('invalid_table').selectAll().execute()

​
Column Reference Validation

Columns must exist in the selected table(s): 
// ✓ Valid
await db
  .selectFrom('person')
  .select(['first_name', 'age'])
  .execute()

// ✗ Error: Type '"invalid_column"' is not assignable
await db
  .selectFrom('person')
  .select(['first_name', 'invalid_column'])
  .execute()

​
JOIN Type Safety

After a join, columns from both tables are available: 
const result = await db
  .selectFrom('person')
  .innerJoin('pet', 'pet.owner_id', 'person.id')
  .select([
    'person.first_name', // ✓ Available
    'pet.name',          // ✓ Available
    'person.age',        // ✓ Available
    'pet.species'        // ✓ Available
  ])
  .execute()

// Type: { first_name: string; name: string; age: number; species: string }[]

​
Join Reference Validation

// ✓ Valid - columns exist in both tables
await db
  .selectFrom('person')
  .innerJoin('pet', 'pet.owner_id', 'person.id')
  .selectAll()
  .execute()

// ✗ Error - invalid column reference
await db
  .selectFrom('person')
  .innerJoin('pet', 'pet.invalid_column', 'person.id')
  .selectAll()
  .execute()

​
WHERE Clause Type Safety

WHERE clauses are validated against available columns and their types: 
// ✓ Valid - comparing number to number
await db
  .selectFrom('person')
  .where('age', '>', 18)
  .selectAll()
  .execute()

// ✓ Valid - comparing string to string
await db
  .selectFrom('person')
  .where('first_name', '=', 'Jennifer')
  .selectAll()
  .execute()

// ✗ Error - comparing number to string
await db
  .selectFrom('person')
  .where('age', '=', 'invalid') // Type error
  .selectAll()
  .execute()

​
INSERT Type Safety

Inserted values must match the table’s Insertable type: 
interface PersonTable {
  id: Generated<number>  // Optional in insert
  first_name: string     // Required
  last_name: string | null  // Can be null
  age: number            // Required
}

// ✓ Valid
await db.insertInto('person').values({
  first_name: 'Jennifer',
  last_name: 'Aniston',
  age: 40
}).execute()

// ✓ Valid - id is optional (Generated)
await db.insertInto('person').values({
  first_name: 'Arnold',
  last_name: null,
  age: 70
}).execute()

// ✗ Error - missing required field 'first_name'
await db.insertInto('person').values({
  last_name: 'Aniston',
  age: 40
}).execute()

// ✗ Error - wrong type for 'age'
await db.insertInto('person').values({
  first_name: 'Jennifer',
  last_name: 'Aniston',
  age: 'forty' // Type error
}).execute()

​
UPDATE Type Safety

Updated values must match the table’s Updateable type: 
interface PersonTable {
  id: Generated<number>
  first_name: string
  last_name: string | null
  created_at: ColumnType<Date, Date, never> // Cannot be updated
}

// ✓ Valid
await db.updateTable('person')
  .set({ age: 41 })
  .where('id', '=', 1)
  .execute()

// ✓ Valid - all fields are optional in updates
await db.updateTable('person')
  .set({ first_name: 'Jenny' })
  .where('id', '=', 1)
  .execute()

// ✗ Error - created_at cannot be updated (type is 'never')
await db.updateTable('person')
  .set({ created_at: new Date() })
  .where('id', '=', 1)
  .execute()

​
Return Type Inference

Return types change based on execution method: 
interface Person {
  id: number
  first_name: string
  last_name: string | null
  age: number
}

// Type: Person[]
const all = await db
  .selectFrom('person')
  .selectAll()
  .execute()

// Type: Person | undefined
const first = await db
  .selectFrom('person')
  .selectAll()
  .executeTakeFirst()

// Type: Person (or throws)
const firstOrThrow = await db
  .selectFrom('person')
  .selectAll()
  .executeTakeFirstOrThrow()

if (first) {
  first.first_name // ✓ Type narrowing works
}

​
Expression Builder Type Safety

The expression builder provides type-safe access to columns in callbacks: 
await db
  .selectFrom('person')
  .select((eb) => [
    'id',
    'first_name',
    eb.fn.count('id').as('person_count')
  ])
  .where((eb) => eb.or([
    eb('age', '<', 18),
    eb('age', '>', 65)
  ]))
  .execute()
The eb parameter knows which columns are available in the current context: 
await db
  .selectFrom('person')
  .innerJoin('pet', 'pet.owner_id', 'person.id')
  .where((eb) => eb.or([
    eb('person.age', '>', 50),  // ✓ Valid
    eb('pet.species', '=', 'dog') // ✓ Valid
  ]))
  .selectAll()
  .execute()

​
Type Narrowing with Generics

Kysely’s query builders are generic, allowing for composable, type-safe functions: 
function withMinAge<DB, TB extends keyof DB, O>(
  qb: SelectQueryBuilder<DB, TB, O>,
  minAge: number
) {
  return qb.where('age', '>=', minAge)
}

const adults = await withMinAge(
  db.selectFrom('person').selectAll(),
  18
).execute()

​
Dynamic Table Access

For runtime-dynamic table access, use db.dynamic.ref(): 
const tableName = 'person' as string // Runtime value
const columnName = 'first_name' as string

await db
  .selectFrom(db.dynamic.ref(tableName))
  .select([db.dynamic.ref(columnName)])
  .execute()

// Note: This bypasses type checking!
Using db.dynamic bypasses Kysely’s type safety. Only use it when absolutely necessary.

​
Compile-Time vs Runtime

Kysely provides compile-time type safety. Type information is erased at runtime: 
// TypeScript compilation:
// ✓ Type checks pass, everything is valid

const query = db
  .selectFrom('person')
  .select(['first_name', 'age'])
  .where('age', '>', 18)

// Runtime execution:
// If 'person' table doesn't exist, you'll get a runtime SQL error
// Kysely cannot validate your database schema at runtime
Use database migration tools and testing to ensure your TypeScript types match your actual database schema.

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Best Practices

Keep your database interface in sync with your actual database schema using migrations or code generation tools.
Use Selectable, Insertable, and Updateable type helpers to extract operation-specific types from your table interfaces.
Avoid using any or type assertions. If you need dynamic queries, use db.dynamic sparingly and document why it’s necessary.
Kysely’s type safety is structural, not nominal. Two tables with identical column types are considered compatible.

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