1. A data type is a set of values that can be represented.
2. A value is either a NULL value (undefined value), or the special NULL value, or a non-NULL value.
3. The NULL value is a special value. The comparison of the NULL value with all values is undefined.
4. A special NULL value is a special value which may occur in arithmetical operations when these lead to an overflow or a division by 0. The comparison of a special NULL value with any value is always undefined.
5. A non-NULL value is a character string, a number, a date value, a time value, a timestamp value, or a value of a LONG column.
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