ac05960_.htm |
Last updated Friday, 21 July 2017All ESQL data types (except REFERENCE) support the concept of the null value. A value of null means that the value is unknown, undefined, or uninitialized. Null values can arise when you refer to message fields that do not exist, access database columns for which no data has been supplied, or use the keyword NULL, which supplies a null literal value.
Null is a distinct state and is not the same as any other value. In particular, for integers it is not the same thing as the value 0 and for character variables it is not the same thing as a string of zero characters. The rules of ESQL arithmetic take null values into account, and you are typically unaware of their existence. Generally, but not always, these rules mean that, if any operand is null, the result is null.
If an expression returns a null value its data type is not, in general, known. All null values, whatever their origin, are therefore treated equally.
This can be regarded as their belonging to the data type NULL , which is a data type that can have just one value, null.
An expression always returns NULL if any of its elements are NULL.
To test whether a field contains a null value, use the IS operator described in Operator=.
SET OutputRoot.XMLNS.Msg.Data.Name = NULL; -- this deletes the fieldSET OutputRoot.XMLNS.Msg.Data.Name VALUE = NULL;
-- this assigns a NULL value to a field without deleting it