Database

We will use the database, whose schema and data import commands are given here:

You can import this database by copy pasting the commands on that page.

Optional-match

OPTIONAL MATCH is another clause where you define a pattern to find in the database. The difference between MATCH is that if the system cannot match a pattern defined by OPTIONAL MATCH, it will set the values in the variables defined only in the OPTIONAL MATCH, to NULL. If you consider the patterns defined in a query that combines MATCH and OPTIONAL MATCH, you can understand the semantics as follows: perform the join inside MATCH and store it as an intermediate table L; (ii) perform the join inside the OPTIONAL MATCH and store it as an intermediate table R. Then take the left outer join of L and R (where L is on the left) on their common variables (i.e., L ⟕ R in relational algebra).

For example, the following query returns the followees of each user or NULL if a user doesn’t follow anyone.

Query:

MATCH (u:User)
OPTIONAL MATCH (u)-[:Follows]->(u1:User)
RETURN u.name, u1.name;

Result:

---------------------
| u.name  | u1.name |
---------------------
| Adam    | Karissa |
---------------------
| Adam    | Zhang   |
---------------------
| Karissa | Zhang   |
---------------------
| Zhang   | Noura   |
---------------------
| Noura   |         |
---------------------

View example in Colab.