NULL Values

NULL values represent missing unknown data.
By default, a table column can hold NULL values.
It is not possible to compare NULL and 0; they are not equivalent.

To select NULL Values:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NULL

MySQL does have an ISNULL() function. However, it works a little bit different from Microsoft’s ISNULL() function.
In MySQL we can use the IFNULL() function, like this:

SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
FROM Products

OR

SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
FROM Products

To select only the records with no NULL values:

SELECT LastName,FirstName,Address FROM Persons
WHERE Address IS NOT NULL