Some notes about null
Take a look at the examples: http://dbtips.wordpress.com/2009/04/26/a-simple-sql-weakness-with-the-intersect-operation/
The following are some practical issues related to what Mr. C.J. Date said about null. Let’s analyse the results in a couple of cases.Consider that we have a couple of attributes att1 and att2.
Case 1 (simple conditions in the example):
att1 = 3 and att2 = null
if we have the condition att1 = att2, the answer is the same as false. But, is the value 3 different from null?, not neccesarily, attibute att2 could be 3, then the correct answer should be unknown.
Case 2 (Intersect in the example):
The intersection of att1 = null in one table and the same att1 = null in the other table.
In this case of the Intersect, null is considered a value, that is why the intersection take place. But null is not a value is a symbol that represent unknown or some other meaning (soon an analysis of this aspect), that`s not correct, because the att1 value from one table could or could not be the same as att1 from the other table.
A detailed explanation is found in my book:
Amazon.com: http://tinyurl.com/amnz-sqlbook
