Query variables in GraphQL - Step-by-Step Execution

Concept Flow - Query variables

Define Query with Variables

↓

Send Query + Variables

↓

Server Receives Query

↓

Server Substitutes Variables

↓

Execute Query with Values

↓

Return Result to Client

This flow shows how GraphQL queries use variables: define variables, send them with the query, server substitutes them, executes, and returns results.

Execution Sample

GraphQL

query GetUser($id: ID!) {
  user(id: $id) {
    name
    email
  }
}

variables: {"id": "123"}

This query fetches a user by ID using a variable $id, which is provided separately in the variables object.

Execution Table

Step	Action	Query Part	Variable State	Server Action	Output
1	Receive query and variables	query GetUser($id: ID!) {...}	{"id": "123"}	Parse query and variables	Ready to substitute variables
2	Substitute variable $id	user(id: $id)	{"id": "123"}	Replace $id with "123"	user(id: "123")
3	Execute query	user(id: "123") { name, email }	{"id": "123"}	Fetch user with id 123	{"name": "Alice", "email": "[e-mail]"}
4	Return result	Full query result	{"id": "123"}	Send data back to client	{"data": {"user": {"name": "Alice", "email": "[e-mail]"}}}
5	End	N/A	N/A	Query complete	Execution stops

💡 Query completes after returning user data for id 123

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
$id	undefined	"123"	"123"	"123"

Key Moments - 3 Insights

Why do we define variables separately from the query?

Variables are defined separately so the query can be reused with different values without rewriting the query text, as shown in execution_table step 1 and 2.

What happens if a variable is missing when the query runs?

If a required variable like $id is missing, the server cannot substitute it (see step 2), so the query will fail before execution.

How does the server know which value to use for $id?

The server uses the variables object sent alongside the query (step 1), matching variable names to values for substitution (step 2).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what value is $id substituted with?

A "123"

B "user"

C undefined

D "name"

💡 Hint

Check the 'Variable State' and 'Server Action' columns at step 2 in the execution_table.

At which step does the server fetch user data from the database?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Server Action' column describing 'Fetch user with id 123' in the execution_table.

If the variable $id was missing, what would happen at step 2?

A The server substitutes $id with null and continues

B The server fails to substitute $id and query execution stops

C The server substitutes $id with an empty string

D The server ignores $id and fetches all users

💡 Hint

Refer to key_moments about missing variables and execution_table step 2.

Concept Snapshot

GraphQL Query Variables:
- Define variables in query signature: query Name($var: Type!)
- Send variables separately as JSON
- Server substitutes variables before execution
- Allows query reuse with different inputs
- Missing required variables cause errors

Full Transcript

This visual execution shows how GraphQL query variables work. First, the client defines a query with variables like $id. Then it sends the query text plus a variables object with actual values. The server receives both, parses them, and substitutes the variables in the query with the provided values. After substitution, the server executes the query using those values, fetches data, and returns the result to the client. If a required variable is missing, the server cannot substitute it and the query fails. This process allows queries to be reused with different inputs easily.

Practice

1. What is the main purpose of using query variables in GraphQL?

easy

A. To create new types in the GraphQL schema

B. To define the schema of the GraphQL server

C. To store query results on the client side

D. To reuse the same query with different input values

Solution

1. Step 1: Understand what query variables do

   Query variables allow you to write a query once and provide different inputs each time you run it.

2. Step 2: Compare options with this purpose

   Only To reuse the same query with different input values describes reusing queries with different inputs, which matches the purpose of query variables.

3. Final Answer:

   To reuse the same query with different input values -> Option D

4. Quick Check:

   Query variables = reuse queries with different inputs [OK]

Hint: Variables let you change inputs without rewriting queries [OK]

Common Mistakes:

• Confusing variables with schema definitions
• Thinking variables store data permanently
• Mixing variables with client-side caching

2. Which of the following is the correct way to declare a variable in a GraphQL query?

easy

A. query getUser($id: ID!) { user(id: $id) { name } }

B. query getUser(id: $ID) { user(id: id) { name } }

C. query getUser($id) { user(id: id) { name } }

D. query getUser { user(id: $id) { name } }

Solution

1. Step 1: Identify correct variable syntax

   Variables must be declared with a $ sign and a type, e.g., $id: ID! inside parentheses after the query name.

2. Step 2: Check each option

   query getUser($id: ID!) { user(id: $id) { name } } correctly declares $id: ID! and uses $id inside the query. Others miss type, $ sign, or parentheses.

3. Final Answer:

   query getUser($id: ID!) { user(id: $id) { name } } -> Option A

4. Quick Check:

   Variable declaration = $name: Type! in parentheses [OK]

Hint: Variables need $ and type declared after query name [OK]

Common Mistakes:

• Omitting the variable type
• Using variable without $ inside query
• Not placing variables in parentheses after query name

3. Given the query:

query getBook($bookId: ID!) { book(id: $bookId) { title author } }

and variables:

{ "bookId": "123" }

What will the query return if the book with ID 123 has title "GraphQL Guide" and author "Anna"?

medium

A. { "data": { "book": null } }

B. { "data": { "book": { "title": "GraphQL Guide", "author": "Anna" } } }

C. SyntaxError: Variable not defined

D. { "errors": [ { "message": "Book not found" } ] }

Solution

1. Step 1: Understand the query and variables

   The query asks for a book by ID using variable $bookId. The variable value is "123".

2. Step 2: Match variable value to data

   The book with ID "123" exists with title "GraphQL Guide" and author "Anna". So the query returns this data.

3. Final Answer:

   { "data": { "book": { "title": "GraphQL Guide", "author": "Anna" } } } -> Option B

4. Quick Check:

   Variable value matches book data = correct result [OK]

Hint: Variables fill query inputs to get matching data [OK]

Common Mistakes:

• Assuming variable names are optional
• Expecting errors when data exists
• Confusing null with errors

4. Identify the error in this GraphQL query using variables:

query getUser($userId: ID) { user(id: userId) { name } }

medium

A. Variable userId inside query should have a $ sign

B. Variable $userId is missing the non-null type indicator

C. Variables must be declared outside the query

D. Query name cannot have variables declared

Solution

1. Step 1: Check variable usage inside query

   The variable is declared as $userId but inside the query it is used as userId without the $ sign.

2. Step 2: Correct usage of variables

   Variables must be referenced with the $ sign inside the query fields, so it should be id: $userId.

3. Final Answer:

   Variable userId inside query should have a $ sign -> Option A

4. Quick Check:

   Use $ before variable inside query fields [OK]

Hint: Always use $ before variable inside query body [OK]

Common Mistakes:

• Forgetting $ before variable in query
• Thinking variables declared outside query
• Confusing variable declaration syntax

5. You want to write a GraphQL query to fetch a list of books filtered by optional author name using variables. Which query correctly declares the variable and uses it to filter books only if the variable is provided?

hard

A. query getBooks($author: String!) { books(author: $author) { title } }

B. query getBooks { books(author: $author) { title } }

C. query getBooks($author: String) { books(author: $author) { title } }

D. query getBooks($author) { books(author: author) { title } }

Solution

1. Step 1: Understand optional variable declaration

   To make a variable optional, declare it without the exclamation mark, e.g., $author: String.

2. Step 2: Check variable usage in query

   Use $author inside the query to filter books. query getBooks($author: String) { books(author: $author) { title } } correctly declares and uses the optional variable.

3. Step 3: Review other options

   query getBooks($author: String!) { books(author: $author) { title } } makes variable required with !, C uses variable without declaration, D misses type and uses variable incorrectly.

4. Final Answer:

   query getBooks($author: String) { books(author: $author) { title } } -> Option C

5. Quick Check:

   Optional variable = no ! and use $ in query [OK]

Hint: Optional variables omit ! and use $ inside query [OK]

Common Mistakes:

• Making optional variables required with !
• Using variables without declaration
• Forgetting $ in query usage