Also, remember that NULL is not equal to anything (even NULL) so it always fails any comparison with a value. So a NULL value in your table column will return false for any comparisons, except when you use the operators "IS NULL" (returns true if value is NULL) or "IS NOT NULL" (returns true if value is anything except NULL).
Depending on your database, you can also use the NVL() function to handle NULL values in your comparisons etc. NVL(input, output) means that if the input value IS NULL, then the function should return the output value instead, otherwise it just returns the input value. For example:
WHERE NVL(price, 0) = 0
This will return the rows where price is a non-NULL value and equal to 0, or where price is NULL (because the NVL function converts the NULLs to 0 before comparing them).