weirdo NULL handling in SQL

Conisder following SQL statement
SELECT * FROM sometable WHERE num <> 1;
If you expected this to return rows where value of num is null, then you are wrong. Similarly
SELECT * FROM sometable WHERE LENGTH(my_col) < 20
above will not return rows with my_col having null values.

This is bcoz SQL don't consider null as a value and since null is not a value it cannot be compared with a value. This is the diffrence between Null and either 0 (zero) or an empty string (a string value with a length of zero, represented in SQL as '' ). While null indicates the absence of any value, the empty string and numerical zero both represent actual values. So be careful while you are writing "less than" or "not equal to" queries on nullable column.

Got some more time read this on wikipedia.