SQL Reference

Result columns do not allow null values if they are derived from:

A column that does not allow null values
A constant
The COUNT or COUNT_BIG function
A host variable that does not have an indicator variable
A scalar function or expression that does not include an operand that allows nulls.

Result columns allow null values if they are derived from:

Any column function except COUNT or COUNT_BIG
A column that allows null values
A scalar function or expression that includes an operand that allows nulls
A NULLIF function with arguments containing equal values.
A host variable that has an indicator variable.
A result of a set operation if at least one of the corresponding items in the select list is nullable.
An arithmetic expression or view column that is derived from an arithmetic expression and the database is configured with DFT_SQLMATHWARN set to yes
A dereference operation.

If the AS clause is specified, the name of the result column is the name specified on the AS clause.
If the AS clause is not specified and the result column is derived from a column, then the result column name is the unqualified name of that column.
If the AS clause is not specified and the result column is derived using a dereference operation, then the result column name is the unqualified name of the target column of the dereference operation.
All other result column names are unnamed. (41)

Footnotes:

(41) The system assigns temporary numbers (as character strings) to these columns. These system assigned names cannot be used in an order-by clause.

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