Questioned Paths

Overview

A questioned path makes a path pattern optional — it matches zero or one occurrence. This is useful when you want to include a connection in a pattern but it may not always exist.

A questioned path is written by appending ? to a parenthesized path pattern expression.

Syntax
<questioned path pattern> ::=
  "(" <path pattern expression> ")" "?"

When the path exists, it is included in the result; when it doesn't, the match still succeeds with the optional part absent.

Difference from {0,1}

Although ? and {0,1} both match zero or one occurrence, they differ in how variables are exposed:

• {0,1} (quantified path): Variables declared inside become group variables — they are exposed as lists (e.g., a single-element list or an empty list).
• ? (questioned path): Variables declared inside remain singletons — they keep their original type but become conditional singletons that are null when the optional path is absent.

Example Graph

GQL
INSERT (jack:User {_id: "U01", name: "Jack"}),
       (mike:User {_id: "U02", name: "Mike"}),
       (c1:Device {_id: "Comp1"}),
       (c2:Device {_id: "Comp2"}),
       (c3:Device {_id: "Comp3"}),
       (c4:Device {_id: "Comp4"}),
       (jack)-[:Owns]->(c1),
       (mike)-[:Owns]->(c4),
       (c1)-[:Flows {packets: 20}]->(c2),
       (c1)-[:Flows {packets: 30}]->(c4),
       (c2)-[:Flows {packets: 34}]->(c3),
       (c2)-[:Flows {packets: 12}]->(c4),
       (c3)-[:Flows {packets: 74}]->(c4)

Examples

Find all devices reachable from Comp1 with an optional second hop:

GQL
MATCH (d1 {_id: 'Comp1'})->(d2:Device)(->(d3:Device))?
RETURN d1._id AS start, d2._id AS firstHop, d3._id AS secondHop

Result:

When the optional path matches, d3 contains the device. When it doesn't, d3 is null. This is because ? exposes variables as conditional singletons rather than group variables.