Last Updated: 3/10/2026

kysely

• Expression

Interface Expression

Expression represents an arbitrary SQL expression with a type.

Most Kysely methods accept instances of Expression and most classes like SelectQueryBuilder and the return value of the sql template tag implement it.

Examples

import { type  Expression, sql } from  'kysely' const  exp1: Expression< string> = sql< string>`CONCAT('hello', ' ', 'world')` const  exp2: Expression<{ first_name: string }> = db. selectFrom('person'). select('first_name')

You can implement the Expression interface to create your own type-safe utilities for Kysely.

interface Expression<T> {
    get expressionType(): undefined | T;
    toOperationNode(): OperationNode;
}

Type Parameters

• T

Hierarchy (View Summary)

• OperationNodeSource
  • Expression
    • AliasableExpression

Index

Accessors

expressionType

Methods

toOperationNode

Accessors

expressionType

• get expressionType(): undefined | T

  All expressions need to have this getter for complicated type-related reasons. Simply add this getter for your expression and always return undefined from it:

  Examples

  import { type  Expression, type  OperationNode, sql } from  'kysely' class  SomeExpression< T> implements  Expression< T> { get  expressionType(): T  |  undefined { return  undefined }  toOperationNode(): OperationNode { return  sql `some sql here`. toOperationNode() }}

  The getter is needed to make the expression assignable to another expression only if the types T are assignable. Without this property (or some other property that references T), you could assing Expression to Expression.

  Returns undefined | T

Methods

toOperationNode

• toOperationNode(): OperationNode

  Creates the OperationNode that describes how to compile this expression into SQL.

  Examples

  If you are creating a custom expression, it’s often easiest to use the sql template tag to build the node:

  import { type  Expression, type  OperationNode, sql } from  'kysely' class  SomeExpression< T> implements  Expression< T> { get  expressionType(): T  | undefined { return  undefined }  toOperationNode(): OperationNode { return  sql `some sql here`. toOperationNode() }}

  Returns OperationNode

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ExamplesAccessors

expressionType

• Examples

Methods

toOperationNode

• Examples