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:

class SomeExpression<T> implements Expression<T> {
  get expressionType(): T | undefined {
    return undefined
  }
}

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<string> to Expression<number>.

Change the output type of the expression.

This method call doesn't change the SQL in any way. This methods simply returns a copy of this OrWrapper with a new output type.

Returns an aliased version of the expression.

In addition to slapping as "the_alias" to the end of the SQL, this method also provides strict typing:

const result = await db
  .selectFrom('person')
  .select(eb =>
    eb('first_name', '=', 'Jennifer')
      .or('first_name', '=', 'Sylvester')
      .as('is_jennifer_or_sylvester')
  )
  .executeTakeFirstOrThrow()

// `is_jennifer_or_sylvester: SqlBool` field exists in the result type.
console.log(result.is_jennifer_or_sylvester)

The generated SQL (PostgreSQL):

select "first_name" = $1 or "first_name" = $2 as "is_jennifer_or_sylvester"
from "person"

Returns an aliased version of the expression.

In addition to slapping as "the_alias" at the end of the expression, this method also provides strict typing:

const result = await db
  .selectFrom('person')
  .select((eb) =>
    // `eb.fn<string>` returns an AliasableExpression<string>
    eb.fn<string>('concat', ['first_name' eb.val(' '), 'last_name']).as('full_name')
  )
  .executeTakeFirstOrThrow()

// `full_name: string` field exists in the result type.
console.log(result.full_name)

The generated SQL (PostgreSQL):

select
  concat("first_name", $1, "last_name") as "full_name"
from
  "person"

You can also pass in a raw SQL snippet (or any expression) but in that case you must provide the alias as the only type argument:

const values = sql<{ a: number, b: string }>`(values (1, 'foo'))`

// The alias is `t(a, b)` which specifies the column names
// in addition to the table name. We must tell kysely that
// columns of the table can be referenced through `t`
// by providing an explicit type argument.
const aliasedValues = values.as<'t'>(sql`t(a, b)`)

await db
  .insertInto('person')
  .columns(['first_name', 'last_name'])
  .expression(
    db.selectFrom(aliasedValues).select(['t.a', 't.b'])
  )

The generated SQL (PostgreSQL):

insert into "person" ("first_name", "last_name")
from (values (1, 'foo')) as t(a, b)
select "t"."a", "t"."b"

Combines this and another expression using OR.

See ExpressionWrapper.or for examples.

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

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

class SomeExpression<T> implements Expression<T> {
  toOperationNode(): OperationNode {
    return sql`some sql here`.toOperationNode()
  }
}