If a column's field has NULL value then it means, it is blank. This field has no value.
When a column's value is NULL then it can't be determined by an eqal to (=) operator.
The way of identifying NULL values is, you need to use IS NULL or IS NOT NULL statement.
NOT NULL in SQL
SELECT * FROM result WHERE marks IS NULL;This query will display all columns where price will be null.
IS NOT NULL in SQL
SELECT * FROM result WHERE marks IS NOT NULL;This query will display all columns where price will not be null.
NULL values are filtered by a WHERE query.
Replace NULL values into specific values-
SQL provides a function coalesce() to replace null values with another value. By adding a placeholder like N/A, NONE, etc. you can make your report more meaningful to people.
SELECT students,coalesce(marks, 0) FROM result;