Intermediate Cypher-1

Let’s talk about aliases in Neo4j Cypher.

Aliases are just variable names you assign to nodes, relationships, or returned values so you can refer to them later in the query.

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🔹 1. Aliases for Nodes

MATCH (u:User)
RETURN u.name, u.role

Here:

• u is the alias (variable) for each User node.

• You can access its properties as u.name, u.role.

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🔹 2. Aliases for Relationships

MATCH (u:User)-[r:WORKS_ON]->(p:Project)
RETURN u.name, p.title, r.since

Here:

• u → alias for User

• p → alias for Project

• r → alias for WORKS_ON relationship

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🔹 3. Aliases in RETURN

You can rename columns in the output (like SQL AS).

MATCH (u:User)
RETURN u.name AS userName, u.role AS userRole

✅ The result will show columns userName and userRole instead of u.name and u.role.

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🔹 4. Aliases in Aggregations

MATCH (u:User)-[:WORKS_ON]->(p:Project)
RETURN p.title AS projectName, count(u) AS teamSize

✅ The output columns are projectName and teamSize.

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🔹 5. Aliases with WITH (for chaining queries)

MATCH (u:User)-[:WORKS_ON]->(p:Project)
WITH p.title AS projectName, count(u) AS teamSize
WHERE teamSize > 2
RETURN projectName, teamSize

• First part: create aliases projectName and teamSize

• Then filter using those aliases.

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⚡ Key Idea:

• Inside patterns (u:User) → alias for node

• Inside RETURN/WITH → alias for results

MERGE vs CREATE

Perfect 👌 this is one of the most important differences to understand in Neo4j. Let’s break it down step by step with examples.

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🔹 CREATE

• Always creates new nodes or relationships, even if an identical one already exists.

• It never checks for duplicates.

Example

CREATE (u:User {name: "Alice"})
CREATE (u:User {name: "Alice"})

👉 You now have two different Alice nodes, even though they look the same.

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🔹 MERGE

• Think of it as "match if exists, otherwise create".

• It searches for a node/relationship that matches the entire pattern.

• If it finds one → reuses it.

• If it doesn’t → creates it.

Example

MERGE (u:User {name: "Alice"})
MERGE (u:User {name: "Alice"})

👉 Only one Alice node exists, because the second MERGE matches the first one.

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🔹 Important Detail

MERGE matches the exact properties you give.
So this can create duplicates if you’re not careful:

MERGE (u:User {name: "Alice"})
MERGE (u:User {name: "Alice", role: "Developer"})

👉 Two Alice nodes get created:

• First has {name: "Alice"}

• Second has {name: "Alice", role: "Developer"}

✅ To avoid this, you can match on fewer properties and then set others:

MERGE (u:User {name: "Alice"})
ON CREATE SET u.role = "Developer"
ON MATCH SET u.lastSeen = date()

Let’s go over how to update data in Neo4j using SET and REMOVE.

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🔹 1. SET – Add or Update Properties

• SET is used to add new properties or update existing ones on nodes/relationships.

• If the property exists → it’s updated.

• If it doesn’t exist → it’s created.

Example – Update a User’s role

MATCH (u:User {name: "Alice"})
SET u.role = "Developer"

👉 If Alice has no role, it will be created. If she already has one, it will be replaced.

Example – Add multiple properties

MATCH (u:User {name: "Alice"})
SET u.location = "India", u.active = true

Example – Copy all properties from a map

MATCH (u:User {name: "Alice"})
SET u += {department: "Engineering", level: 2}

👉 Adds department and level without removing existing ones.

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🔹 2. REMOVE – Delete Properties or Labels

• REMOVE is used to delete a property from a node/relationship.

• It can also remove labels from nodes.

Example – Remove a property

MATCH (u:User {name: "Alice"})
REMOVE u.location

👉 Deletes the location property.

Example – Remove a label

MATCH (u:User {name: "Alice"})
REMOVE u:User

👉 Alice is no longer a User node (but still exists as a node in the graph).

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🔹 3. SET vs REMOVE – Quick Comparison

Operation	Command	Effect
Add/update property	SET u.role = "Admin"	Adds or updates role
Add multiple props	SET u += {a:1, b:2}	Adds both a and b
Remove property	REMOVE u.role	Deletes role property
Remove label	REMOVE u:User	Removes User label

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⚡ Tip:

• Use SET when you want to add or update information.

• Use REMOVE when you want to delete properties or labels but keep the node/relationship itself.

In Cypher, when you use

SET u += {department: "Engineering", level: 2}

it works like a merge (upsert) of properties, not like replacing the whole property map.

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🔹 What actually happens:

• If a property does not exist → it gets added.

• If a property already exists → its value gets overwritten (updated).

• Other properties stay unchanged.

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Example:

Suppose Alice has this node:

{
  "name": "Alice",
  "department": "Finance",
  "location": "India"
}

After running:

MATCH (u:User {name: "Alice"})
SET u += {department: "Engineering", level: 2}

👉 Result:

{
  "name": "Alice",
  "department": "Engineering",   // ✅ updated
  "location": "India",           // ✅ unchanged
  "level": 2                     // ✅ new property added
}

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🔹 Key difference vs direct SET

If you did:

SET u = {department: "Engineering", level: 2}

👉 Then Alice’s node would keep only those two properties, and name + location would be lost.

So += is like safe update/merge, while = is replace everything.

deleting in Neo4j has two flavors, and the difference is very important.

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🔹 DELETE

• Removes nodes or relationships, but only if the node has no relationships.

• If a node is still connected, Neo4j will throw an error.

Example

// Suppose Alice works on ProjectX
MATCH (u:User {name: "Alice"})
DELETE u

👉 ❌ Error: Cannot delete node <id> because it still has relationships.
Because Alice is linked to ProjectX.

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🔹 DETACH DELETE

• Deletes a node and all its relationships in one step.

• Think of it as force delete.

Example

MATCH (u:User {name: "Alice"})
DETACH DELETE u

👉 ✅ Alice is deleted, and her relationships to projects, apps, etc. are also removed automatically.

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🔹 Deleting Relationships

You can delete relationships separately too:

MATCH (u:User {name: "Alice"})-[r:WORKS_ON]->(p:Project)
DELETE r

👉 Only the relationship is deleted, Alice and ProjectX remain.

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⚡ Quick Summary

Command	What it does
DELETE	Deletes a node/relationship if no other relationships exist
DETACH DELETE	Deletes a node and all its connected relationships
DELETE r	Deletes a relationship only