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.

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

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SELECT user_id
FROM course_feedback
WHERE feedback_text IS NULL;

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).

Assume we have the following table:

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

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

The query returns the following:

Result:

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