# GraphQL SDL — Schema Definition Language

URL: https://www.prisma.io/blog/graphql-sdl-schema-definition-language-6755bcb9ce51
Published: 2017-05-01
Authors: Nikolas Burk
Description: Learn the GraphQL Schema Definition Language (SDL) and how it is used to define types, fields, arguments, and relationships in GraphQL schemas.

GraphQL has its own language to write GraphQL Schemas: The GraphQL Schema Definition Language (SDL). SDL is simple and
intuitive to use while being extremely powerful and expressive.

## What is a GraphQL Schema Definition? [#what-is-a-graphql-schema-definition]

A GraphQL Schema Definition is the most concise way to specify a [GraphQL schema](https://www.prisma.io/blog/graphql-server-basics-the-schema-ac5e2950214e). The syntax is well-defined and [are part of the official GraphQL specification](https://github.com/facebook/graphql/pull/90). Schema Definitions are sometimes referred to as IDL (Interface Definition Language) or SDL (Schema Definition Language).

The GraphQL schema for a blogging app could be specified like this:

```graphql
type Post {
  id: String!
  title: String!
  publishedAt: DateTime!
  likes: Int! @default(value: 0)
  blog: Blog @relation(name: "Posts")
}

type Blog {
  id: String!
  name: String!
  description: String
  posts: [Post!]! @relation(name: "Posts")
}
```

The main components of a schema definition are the *types* and their *fields*. Additional information can be provided as custom *directives* like the `@default` value specified for the `likes` field.

### Type [#type]

A type has a name and can implement one or more *interfaces*:

```graphql
type Post implements Item {
  # ...
}
```

## Field [#field]

A field has a name and a type:

```graphql
age: Int
```

The GraphQL spec defines some [built-in](https://spec.graphql.org/June2018/#sec-Scalars) scalar values but more can be defined by a concrete implementation. The built in scalar types are:

* Int
* Float
* String
* Boolean
* ID

In addition to scalar types, a field can use any other type defined in the schema definition.

Non-nullable fields are denoted by an exclamation mark:

```graphql
age: Int!
```

Lists are denoted by square brackets:

```graphql
names: [String!]
```

## Enum [#enum]

An `enum` is a scalar value that has a specified set of possible values:

```graphql
enum Category {
  PROGRAMMING_LANGUAGES
  API_DESIGN
}
```

## Interface [#interface]

In GraphQL an `interface` is a list of fields. A GraphQL type must have the same fields as all the interfaces it implements and all interface fields must be of the same type.

```graphql
interface Item {
  title: String!
}
```

## Schema directive [#schema-directive]

A directive allows you to attach arbitrary information to any other schema definition element. Directives are always placed behind the element they describe:

```graphql
name: String! @defaultValue(value: "new blogpost")
```

Directives don’t have intrinsic meaning. Each GraphQL implementation can define their own custom directives that add new functionality.

GraphQL specifies built-in skip and include directives that can be used to include or exclude specific fields in queries, but these aren't used in the schema language.