DBMS-Null Values
In some cases a particular entity may not have an applicable value for an attribute. For example, the ApartmentNumber attribute of an address applies only to addresses that are in apartment buildings and not to other types of residences, such as single-family homes. Similarly, a CollegeDegrees attribute applies only to persons with college degrees. For such situations, a special value called null is created. An address of a single-family home would have null for its ApartmentNumber attribute, and a person with no college degree would have null for CollegeDegrees. Null can also be used if we do not know the value of an attribute for a particular entity—for example, if we do not know the home phone of "John Smith" in Figure 03.03. The meaning of the former type of null is not applicable, whereas the meaning of the latter is unknown. The unknown category of null can be further classified into two cases. The first case arises when it is known that the attribute value exists but is missing—for example, if the Height attribute of a person is listed as null. The second case arises when it is not known whether the attribute value exists—for example, if the HomePhone attribute of a person is null.
Calling Built-In Applications Using Intents Until this point, you have seen how to call activities within your own application. One of th...