Summary  
This chapter explains how to detect and filter NULL values in SQL and how to use functions like IFNULL, COALESCE, and NULLIF to provide default or fallback values for missing data.

General domain of usage  
Relational database querying

Understanding how SQL handles `NULL` values is essential for writing reliable queries. In SQL, `NULL` represents missing or unknown data. Unlike zero or an empty string, `NULL` is a special marker that means the value does not exist. If you do not handle `NULL` values properly, you might get unexpected results when filtering, aggregating, or performing calculations. This is especially important in real-world databases, where missing information is common.

-- Find employees who do not have a bonus assigned
SELECT name, department
FROM employees
WHERE bonus IS NULL;

-- Find employees who have a bonus assigned
SELECT name, bonus
FROM employees
WHERE bonus IS NOT NULL;

SQL provides several functions to work with `NULL` values effectively. The `IFNULL` function returns a specified value if the expression is `NULL`; otherwise, it returns the expression itself. The `COALESCE` function returns the first non-null value in a list of expressions, making it useful for providing fallback values. The `NULLIF` function compares two expressions and returns `NULL` if they are equal; otherwise, it returns the first expression. These functions help you manage missing data and prevent errors in your queries.

-- Show each employee's bonus, using 1000.00 as a default if bonus is NULL
SELECT name, COALESCE(bonus, 1000.00) AS bonus
FROM employees;

Which function returns the first non-null value?

This beginner course focuses on the essential SQL functions used for data cleaning and transformation. Learners will practice handling missing values, formatting and standardizing data, and applying string, date, numeric, logical, and null handling functions to prepare datasets for analysis. The course provides hands-on challenges directly related to the most common data preparation tasks in SQL.

Understanding Null Functions

1. Which function returns the first non-null value?

2. What does NULLIF do in a query?

3. Fill in the blanks to filter rows where bonus is NULL.