10.8. 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 three-valued logic. Here is the truth table for AND, OR and XOR.

a b a AND b a OR b a XOR b

FALSE

FALSE

FALSE

FALSE

FALSE

FALSE

NULL

FALSE

NULL

NULL

FALSE

TRUE

FALSE

TRUE

TRUE

TRUE

FALSE

FALSE

TRUE

TRUE

TRUE

NULL

NULL

TRUE

NULL

TRUE

TRUE

TRUE

TRUE

FALSE

NULL

FALSE

FALSE

NULL

NULL

NULL

NULL

NULL

NULL

NULL

NULL

TRUE

NULL

TRUE

NULL

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]

TRUE

2 IN [1, NULL, 3]

NULL

2 IN [1, 2, NULL]

TRUE

2 IN [1]

FALSE

2 IN []

FALSE

NULL IN [1,2,3]

NULL

NULL IN [1,NULL,3]

NULL

NULL IN []

FALSE

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)