NULL Handling in SQL

In SQL, NULL represents a missing or unknown value. Unlike 0, an empty string, or false, it indicates the absence of data.

Understanding how to detect and manage NULL values is critical for writing accurate queries.

Checking for NULL

You cannot use = or != to compare against NULL. Instead, use:

• IS NULL: checks if a value is NULL
• IS NOT NULL: checks if a value is not NULL

Find users without feedback

SELECT user_id
FROM course_feedback
WHERE feedback_text IS NULL;

Replacing NULLs: COALESCE and IFNULL

Use COALESCE and IFNULL to replace NULL values with a fallback value.

• COALESCE(value1, value2, ...) returns the first non-null value.
• IFNULL(value, fallback) replaces a NULL with the fallback (only in some SQL engines like MySQL or SQLite).

Example: Replace missing feedback with 'No comment'

Assume we have the following table:

user_id	course_name	feedback_text
1	SQL Basics	Great course!
2	SQL Basics	NULL
3	SQL Basics	NULL

We want to replace missing feedback with 'No comment'.

Replace missing feedback with 'No comment'

SELECT user_id,
  course_name,
  COALESCE(feedback_text, 'No comment') AS comment
FROM course_feedback;

The query returns the following:

Result:

user_id	course_name	comment
1	SQL Basics	Great course!
2	SQL Basics	No comment
3	SQL Basics	No comment

When do I need to handle NULLs?

Ignoring NULLs can lead to unexpected filters, broken conditions, and inaccurate results. Proper handling ensures your queries stay accurate and your reports reliable.

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