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.
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.
§Extends
§Methods
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>.
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()
}
}