Null values are columns in a row which have no value.
If a column in a row has no value, then the column is said to be Null, or to contain Null. Use a Null when the actual value is not known or when a value would not be meaningful.
Nearly all predefined scalar functions return Null when given a Null argument.
Most aggregate functions ignore Nulls. For example, consider a query that averages the five values 500, NULL, NULL, NULL, and 1500. Such a query ignores the Nulls and calculates the average to be (500+1500)/2 = 1000.
To test for Nulls, use IS NULL and IS NOT NULL. Do not use a comparison operator to test whether a value is Null. Since Null represents an unknown value, the result is also unknown, and therefore the result of the comparison is Null.
WHERE x != NULL -- WRONG! WHERE X IS NOT NULL -- RIGHT