9.8. Working with NULL
Introduction to NULL in Cypher
In Cypher, NULL
is used to represent missing or undefined values.
Conceptually, NULL
means “a missing unknown value” and it is treated somewhat differently from other values.
For example getting a property from a node that does not have said property produces NULL
.
Most expressions that take NULL
as input will produce NULL
.
This includes boolean expressions that are used as predicates in the WHERE
clause.
In this case, anything that is not TRUE
is interpreted as being false.
NULL
is not equal to NULL
.
Not knowing two values does not imply that they are the same value.
So the expression NULL
= NULL
yields NULL
and not TRUE
.
Logical operations with NULL
The logical operators (AND
, OR
, XOR
, IN
, NOT
) treat NULL
as the “unknown” value of threevalued logic.
Here is the truth table for AND
, OR
and XOR
.
a  b  a AND b  a OR b  a XOR b 














































The IN operator and NULL
The IN
operator follows similar logic.
If Cypher knows that something exists in a collection, the result will be TRUE
.
Any collection that contains a NULL
and doesn’t have a matching element will return NULL
.
Otherwise, the result will be false.
Here is a table with examples:
Expression  Result 

2 IN [1, 2, 3] 

2 IN [1, 

2 IN [1, 2, 

2 IN [1] 

2 IN [] 







Using ALL
, ANY
, NONE
, and SINGLE
follows a similar rule.
If the result can be calculated definitely, TRUE
or FALSE
is returned.
Otherwise NULL
is produced.
Expressions that return NULL

Getting a missing element from a collection:
[][0]
,head([])

Trying to access a property that does not exist on a node or relationship:
n.missingProperty

Comparisons when either side is
NULL
:1 < NULL

Arithmetic expressions containing
NULL
:1 + NULL

Function calls where any arguments are
NULL
:sin(NULL)