GraphQL - Type System

GraphQL is a strongly typed language. Type System defines various data types that can be used in a GraphQL application. The type system helps to define the schema, which is a contract between client and server. The commonly used GraphQL data types are as follows −
Scalar
Stores a single value
Object
Shows what kind of object can be fetched
Query
Entry point type to other specific types
Mutation
Entry point for data manipulation
Enum
Useful in a situation where you need the user to pick from a prescribed list of options
Scalar types are primitive data types that can store only a single value. The default scalar types that GraphQL offers are −
Int − Signed 32-bit Integer
Int − Signed 32-bit Integer
Float − Signed double precision floating point value
Float − Signed double precision floating point value
String − UTF - 8-character sequence
String − UTF - 8-character sequence
Boolean − True or false
Boolean − True or false
ID − A unique identifier, often used as a unique identifier to fetch an object or as the key for a cache.
ID − A unique identifier, often used as a unique identifier to fetch an object or as the key for a cache.
The syntax for defining a scalar type is as follows −
field: data_type

The snippet given below defines a field named greeting which returns String value.
greeting: String

The object type is the most common type used in a schema and represents a group of fields. Each field inside an object type maps to another type, thereby allowing nested types. In other words, an object type is composed of multiple scalar types or object types.
The syntax for defining an object type is given below −
type object_type_name
{
   field1: data_type
   field2:data_type 
   ....
   fieldn:data_type
}

You can consider the following code snippet −
--Define an object type--

type Student {
   stud_id:ID
   firstname: String
   age: Int
   score:Float
}

--Defining a GraphQL schema--  

type Query
{
   stud_details:[Student]
}

The example given above defines an object data-type Student. The stud_details field in the root Query schema will return a list of Student objects.
A GraphQL query is used to fetch data. It is like requesting a resource in REST-based APIs. To keep it simple, the Query type is the request sent from a client application to the GraphQL server. GraphQL uses the Schema Definition Language (SDL) to define a Query. Query type is one of the many root-level types in GraphQL.
The syntax for defining a Query is as given below −
type Query {
   field1: data_type
   field2:data_type
   field2(param1:data_type,param2:data_type,...paramN:data_type):data_type
}

An example of defining a Query −
type Query  {
   greeting: String
}
Mutations are operations sent to the server to create, update or delete data. These are analogous to the PUT, POST, PATCH and DELETE verbs to call REST-based APIs.
Mutation is one of the root-level data-types in GraphQL. The Query type defines the entry-points for data-fetching operations whereas the Mutation type specifies the entry points for data-manipulation operations.
The syntax for defining a Mutation type is given below −
type Mutation {
   field1: data_type
   field2(param1:data_type,param2:data_type,...paramN:data_type):data_type 
}

For example, we can define a mutation type to add a new Student as below −
type Mutation {
   addStudent(firstName: String, lastName: String): Student
}
An Enum is similar to a scalar type. Enums are useful in a situation where the value for a field must be from a prescribed list of options.
The syntax for defining an Enum type is −
type enum_name{
   value1
   value2
}

Following snippet illustrates how an enum type can be defined −
type Days_of_Week{
   SUNDAY
   MONDAY
   TUESDAY
   WEDNESDAY
   THURSDAY
   FRIDAY
   SATURDAY
}
Lists can be used to represent an array of values of specific type. Lists are defined with a type modifier [] that wraps object types, scalars, and enums.
The following syntax can be used to define a list type −
field:[data_type]

The below example defines a list type todos −
type Query {
   todos: [String]
}
By default, each of the core scalar types can be set to null. In other words, these types can either return a value of the specified type or they can have no value. To override this default and specify that a field must be defined, an exclamation mark (!) can be appended to a type. This ensures the presence of value in results returned by the query.
The following syntax can be used to define a non-nullable field −
field:data_type!

In the below example, stud_id is declared as a mandatory field.
type Student {
   stud_id:ID!
   firstName:String
   lastName:String
   fullName:String
   college:College
}

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