11 Data Independence and Database Views

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Although NULL is a value that can appear in tuples, it is not a constant . We cannot use NULL explicitly as an operand .

!!!!!!=1>1>415 " Let x have the value NULL. Then the value x + 3 is also NULL.

However, NULL + 3 is not a legal SQL expression. Similarly, the value of x = 3 is UNKNOWN because we cannot tell if

the value of x, which is NULL, equals the value 3. However, the comparison NULL = 3 is not correct SQL.

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Nathan Wilson

11 Data Independence and Database Views

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