SQL > NULL
In SQL, NULL means that data does not exist. NULL does not equal to 0 or an empty string. Both 0 and empty string represent a value, while NULL has no value.
Any mathematical operations performed on NULL will result in NULL. For example,
10 + NULL = NULL
Let's take a look at the following example:
Below are the results for each aggregate function:
SELECT SUM (Sales), AVG (Sales), MAX (Sales), MIN (Sales), COUNT (Sales)
Note that the AVG function counts only 3 rows (the NULL row is excluded), so the average is 600 / 3 = 200, not 600 / 4 = 150. The COUNT function also ignores the NULL row, which is why COUNT (Sales) = 3.