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> ")" "?"

The ? quantifier is equivalent to {0,1}. When the path exists, it is included in the result; when it doesn't, the match still succeeds with the optional part 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

Optional Connection

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

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

When the second hop exists, d2 contains the device; when it doesn't, d2 is null.

Optional Ownership

Find all devices and optionally their owners:

GQL
MATCH (d:Device)(()<-[:Owns]-(u:User))?
RETURN d._id AS device, u.name AS owner

Devices without an owner still appear in the results with owner as null.