Neo4j: WITH, Aggregation, DISTINCT

1. Introduction

In Neo4j, Cypher is the query language that allows you to interact with your graph database. This lesson will cover essential components such as WITH, Aggregation, and DISTINCT, which are vital for manipulating and retrieving data efficiently.

2. Key Concepts

• WITH: A clause used to chain multiple parts of a query. It allows you to carry forward results from one part of the query to another.
• Aggregation: Functions that process multiple rows of data and return a single value (e.g., COUNT, SUM, AVG).
• DISTINCT: A keyword used to return unique values, eliminating duplicates in query results.

3. Usage

3.1 Using WITH

The WITH clause is often used to structure complex queries. Here's how it works:

1. Define the initial part of your query.
2. Use WITH to specify the results to pass to the next part of the query.
3. Continue building your query as needed.

3.2 Aggregation Functions

Common aggregation functions include:

• COUNT(): Counts the number of rows.
• SUM(): Adds up numeric values.
• AVG(): Calculates the average of numeric values.
• MIN() and MAX(): Get the smallest and largest values, respectively.

3.3 Using DISTINCT

The DISTINCT keyword is applied to the result set to ensure uniqueness:

Example: RETURN DISTINCT property will return unique property values from the result set.

4. Examples

4.1 Example Query with WITH

MATCH (p:Person)
WITH p.age AS age, COUNT(p) AS count
RETURN age, count
ORDER BY age
            

This query counts the number of people per age and returns the results ordered by age.

4.2 Example with Aggregation

MATCH (m:Movie)
RETURN AVG(m.released) AS averageReleaseYear
            

This query calculates the average release year of movies.

4.3 Example with DISTINCT

MATCH (p:Person)
RETURN DISTINCT p.name
            

This query returns a list of unique names of persons.

5. Best Practices

• Use WITH to limit the result size early, improving performance.
• Always consider using DISTINCT when you expect duplicate results.
• Group your aggregations logically to maintain clarity in your queries.

6. FAQ