June 2014 Archive


Dates and Times

Implementations of SQL normally support dates and times as special data types. These values are often representable in a variety of formats such as 5/14/1948 or 14 May 1948. Here we shall explain only the SQL standard notation, which is very specific about format.

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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