Introduction to NULL Values

In MySQL, NULL is a special marker that represents missing or unknown data. It is not the same as an empty string or zero; instead, it signifies the absence of a value in a column. Understanding how to work with NULL values is important for accurate data representation and querying.


Dealing with NULL in SQL

SQL provides several functions and operators for working with NULL values:

  • IS NULL
    : Tests if a value is NULL.
  • IS NOT NULL
    : Tests if a value is not NULL.
  • COALESCE
    : Returns the first non-NULL value from a list of expressions.

Example: Finding Records with NULL Values

Let's say you have a "students" table with a column named "email," and you want to find students without an email address. You can use the 

IS NULL
operator:

            SELECT student_name
            FROM students
            WHERE email IS NULL;

This query retrieves the names of students without an email address.


Example: Replacing NULL with a Default Value

The 

COALESCE
function can be used to replace NULL values with a default value. For instance, if you want to display "N/A" for students without an email:

            SELECT student_name, COALESCE(email, 'N/A') AS email
            FROM students;

Conclusion

NULL values in MySQL are important for representing missing or unknown data. You've learned about SQL operators and functions for dealing with NULL, including 

IS NULL
and 
COALESCE
. Accurately handling NULL values in your database queries is crucial for maintaining data integrity and producing meaningful results.