To check whether a value is or is not null, use the constructs
expression IS NULL expression IS NOT NULL
or the equivalent, but nonstandard, constructs
expression ISNULL expression NOTNULL
Do not write expression = NULL because NULL is not “equal to” NULL.
The null value represents an unknown value, and it is not known whether two unknown values are equal. This behavior conforms to the SQL standard.
SELECT product_name, unit_price FROM products WHERE supplier_id IS NULL; //Output 0 rows
Meaning, all the rows have their supplier_id.