UNION splices return values that have same alias from two RETURN clauses head-to-tail, and deduplicates the rows united by these return values.

Syntax:
... RETURN <expression1_A> as <alias_A>, <expression1_B> as <alias_B>, ...
UNION
... RETURN <expression2_A> as <alias_A>, <expression2_B> as <alias_B>, ...
Input:

• <expression1>: Return values of the 1st RETURN
• <expression2>: Return values of the 2nd RETURN, should have the same number of return values as the 1st RETURN and the same data structure of each same alias
• <alias>: The alias of return value (different order allowed)

For instance, splice heterologous return values a and b and deduplicate rows:

uncollect [1,2,3] as a
uncollect [3,4,5] as b
return a, b
union
uncollect [1,2] as a
uncollect [3,5] as b
return a, b

Sample graph: (to be used for the following examples)

create().node_schema("student").node_schema("course")
create().node_property(@*, "name").node_property(@student, "age", int32).node_property(@course, "credit", int32)
insert().into(@student).nodes([{_id:"S001", _uuid:1, name:"Jason", age:25}, {_id:"S002", _uuid:2, name:"Lina", age:23}, {_id:"S003", _uuid:3, name:"Eric", age:24}, {_id:"S004", _uuid:4, name:"Emma", age:26}, {_id:"S005", _uuid:5, name:"Pepe", age:24}])
insert().into(@course).nodes([{_id:"C001", _uuid:6, name:"French", credit:4}, {_id:"C002", _uuid:7, name:"Math", credit:5}])
insert().into(@default).edges([{_uuid:1, _from_uuid:1, _to_uuid:6}, {_uuid:2, _from_uuid:2, _to_uuid:6}, {_uuid:3, _from_uuid:3, _to_uuid:6}, {_uuid:4, _from_uuid:2, _to_uuid:7}, {_uuid:5, _from_uuid:3, _to_uuid:7}, {_uuid:6, _from_uuid:4, _to_uuid:7}, {_uuid:7, _from_uuid:5, _to_uuid:7}])

Common Usage

Example: Find students no elder than 24-year-old that select French, also find students no younger than 24-year-old that select Math, return these students after deduplication

n({@course.name == "French"}).e().n({@student.age <= 24} as n) return n.name
union
n({@course.name == "Math"}).e().n({@student.age >= 24} as n) return n.name

Lina
Eric
Pepe
Emma