#           
## Overview
All types in the GraphQL system are **nullable** by default. This means that a type like integer can not only take a number, including `1, 2, 3,...`, as its value but also take `null`, which means no values. At the same time, GraphQL also allows us to make any type **non-null**. This means that the types we make non-null will never return `null`. We always provide some values to the non-null types in GraphQL.
## When to use
When we want to ensure that a particular field always exists, we use **non-null constraints**. It is important to remember that using a non-null field may make it more difficult to extend our GraphQL schema.


## How non-null works
Let’s look at an example. In the following schema, we have a `Hotel` type with a `hotelLocation` field that refers to a `Location` type:


type Hotel {
  hotelName: String!
  rating: Int!
  hotelLocation: Location!
}

type Location {
  address: String!
}

type Query {
  hotel(query: String): [Hotel]
}

`Non-null` fields are those that have an exclamation mark (`!`) next to their type. When we query these fields, they will not return a null value. Here's a query:

hotel {
    hotelName
    hotelLocation {
      address
    }
  }


// no restaurants exist
{ hotels: null } 

// all data present
{
  hotels: [{
    hotelName: "pearl continental",
    hotelLocation: {
      address: "Street 85, San Francisco"
    }
  }]
}

However, we will not be able to obtain the following outcomes:

// hotelName or hotelLocation not be null
// this result would be incorrect
{
  hotel: [{
    hotelName: null,
    hotelLocation: null
  }]
}

To ensure the structure of the response, GraphQL may also ensure the presence of specific fields when queried.

What are non-null constraints in GraphQL?

Ensure specific fields always exist. Non-null types can't return null, enhancing schema reliability.