Null Values

Null Values and Comparisons Involving NULL

SQL allows attributes to have a special value NULL, which is called the null value. There are many different interpretations that can be put on null values. Here are some of the most common:

The Truth-Value UNKNOWN

In Selection in SQL we assumed that the result of a comparison was either TRUE or FALSE, and these truth-values were combined in the obvious way using the logical operators AND, OR and NOT. We have just seen that when NULL values occur, comparisons can yield a third

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