What is a GraphQL schema? Explain scalar and object types.

Question

IT Interview Practice · Accepted Answer

A **schema** is the typed contract of a GraphQL API written in SDL (Schema Definition Language). It declares every type, every field, and the special **root types** — `Query` (read), `Mutation` (write), and optionally `Subscription` (live). The server rejects any request that does not match the schema.

## Scalar vs object types

- **Scalar types** are leaf values that hold actual data: built-ins are `Int`, `Float`, `String`, `Boolean`, and `ID`. You can also define custom scalars like `DateTime`.
- **Object types** group fields together. Their fields can be scalars *or* other object types, which is how you model relationships.

```graphql
scalar DateTime                # a custom scalar

type User {                    # an object type
  id: ID!                      # ! means non-null (always present)
  name: String!
  createdAt: DateTime!
  posts: [Post!]!              # a list of Post objects, non-null list of non-null items
}

type Post {
  id: ID!
  title: String!
  author: User!                # points back to User -> a relationship
}

type Query {                   # root type: the entry points for reads
  user(id: ID!): User          # a field with an argument; returns a User (nullable)
}
```

The `!` marks **non-null**; `[Post!]!` is a non-null list whose items are non-null. Arguments (`id: ID!`) let a field be parameterized.

## Why it matters

The schema is the single source of truth — it drives validation, auto-generated docs, and client codegen. Reading `!` and list types correctly is a common junior screening check, and nullability decisions shape how clients handle errors.

Scalar vs object types

Why it matters

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