All scalar data types in APL have a special value that represents a missing value. This value is called the null value, or null.

​
Null literals

The null value of a scalar type D is represented in the query language by the null literal D(null). The following query returns a single row full of null values: 
print bool(null), datetime(null), dynamic(null), int(null), long(null), real(null), double(null), time(null)

​
Predicates on null values

The scalar function isnull() can be used to determine if a scalar value is the null value. The corresponding function isnotnull() can be used to determine if a scalar value isn’t the null value.

​
Equality and inequality of null values

  • Equality (==): Applying the equality operator to two null values yields bool(null). Applying the equality operator to a null value and a non-null value yields bool(false).
  • inequality(!=): Applying the inequality operator to two null values yields bool(null). Applying the inequality operator to a null value and a non-null value yields bool(true).