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).

We will use the example database for demonstration, whose schema and data import commands are given here.

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:

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