Notice that NULL s are different than a zero - they are cells where data does not exist.

When identifying NULL s in a WHERE clause, we write IS NULL or IS NOT NULL . We don't use = , because NULL isn't considered a value in SQL. Rather, it is a property of the data.

NULLs - Expert Tip

There are two common ways in which you are likely to encounter NULL s:

• NULL s frequently occur when performing a LEFT or RIGHT JOIN . You saw in the last lesson - when some rows in the left table of a left join are not matched with rows in the right table, those rows will contain some NULL values in the result set.
  
  

• NULL s can also occur from simply missing data in our database.