AQL Essentials: Writing Your First Query in ArangoDB

ArangoDB uses a powerful query language called AQL (ArangoDB Query Language), which allows you to retrieve and manipulate data stored in various formats within the database. This post will cover the fundamentals of AQL, including its syntax, basic operations, and practical examples.

Understanding AQL Syntax

AQL is designed to be easy to read and write, resembling SQL while providing more flexibility for multi-model databases. The basic structure of an AQL query includes:

• FOR: Iterates over a collection.
• FILTER: Applies conditions to narrow down results.
• RETURN: Specifies the data to return.

Basic Query Examples

1. Selecting All Documents from a Collection
To retrieve all documents from a collection named users, you would write:

aql
FOR user IN users
  RETURN user

This query iterates through the users collection and returns every document.

2. Filtering Documents
You can filter documents based on specific conditions. For example, to get all users older than 25:

aql
FOR user IN users
  FILTER user.age > 25
  RETURN user

This query only returns documents where the age field is greater than 25.

3. Using Multiple Conditions
You can combine multiple conditions using logical operators:

aql
FOR user IN users
  FILTER user.age > 25 AND user.email != null
  RETURN user

This returns users older than 25 who also have a valid email address.

Advanced Querying Techniques

AQL supports more advanced querying techniques to help you manipulate and retrieve data effectively.

1. Sorting Results
You can sort the results of your queries. For example, to retrieve users sorted by age in descending order:

aql
FOR user IN users
  SORT user.age DESC
  RETURN user

 

2. Limit and Offset
To limit the number of results returned or to paginate through results, use LIMIT and OFFSET:

aql
FOR user IN users
  LIMIT 10 OFFSET 20
  RETURN user

This retrieves ten users, starting from the 21st user in the collection.

3. Projection
You can project specific fields from your documents. For example, to return only the names and emails of users:

aql
FOR user IN users
  RETURN { name: user.name, email: user.email }

Working with Nested Documents

Since ArangoDB stores data in JSON format, you can also query nested documents. For example, if you have a document structure with an address object:

json
{
  "_key": "user1",
  "name": "Alice",
  "address": {
    "city": "Springfield",
    "zip": "62704"
  }
}
 

You can query for users in a specific city:
aql
FOR user IN users
  FILTER user.address.city == "Springfield"
  RETURN user
 

Using Functions in AQL

AQL provides built-in functions that can be useful for various operations.

1. String Functions
For example, you can use the LENGTH function to count the length of strings:
aql
FOR user IN users
  FILTER LENGTH(user.name) > 10
  RETURN user
 

2. Mathematical Functions
You can also perform mathematical operations. To retrieve users whose age is more than the average age:
aql
LET avgAge = (
  FOR user IN users
    RETURN user.age
)
FOR user IN users
  FILTER user.age > AVG(avgAge)
  RETURN user

Graph Queries with AQL

AQL also includes capabilities for querying graph data. Suppose you have a collection of users and edges representing friendships:

To find friends of a specific user:
aql
FOR friend IN 1..1 OUTBOUND "users/user1" friends
  RETURN friend
This query traverses the graph to return all users connected to user1 through the "friends" edges.

Conclusion
AQL is a powerful and flexible query language that allows you to manipulate and retrieve data in ArangoDB. Its syntax is designed to be intuitive and easy to learn, making it suitable for both beginners and experienced developers. In the next post, we will delve into CRUD operations in ArangoDB, covering how to create, read, update, and delete documents effectively.