How to Handle NULL Values in MySQL

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How do I handle NULL values in MySQL queries?

Techniques and functions to detect, compare, and substitute NULL values in MySQL queries.

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Description

Why are NULLs special in MySQL?

NULL represents "unknown" data. Any arithmetic or comparison with NULL returns NULL, so special operators and functions are required to test or replace it.

How do I detect missing data?

Use IS NULL and IS NOT NULL.Ordinary = and != never match NULL.

Example

SELECT id, name FROM Customers WHERE email IS NULL;

How can I safely compare values that may be NULL?

Use NULL-safe equal <=> to return TRUE when both sides are NULL.

Example

SELECT * FROM Products p WHERE p.price <=> NULL; — returns rows where price is NULL.

Which functions replace NULL with defaults?

IFNULL(expr, alt) and COALESCE(expr1, ...) return the first non-NULL argument.

Example

SELECT id, IFNULL(email, 'no-email@example.com') AS safe_email FROM Customers;

How do I prevent division by NULL?

Wrap denominators with COALESCE(denominator,1) to avoid NULL dividers.

When is `NULLIF` useful?

NULLIF(a,b) returns NULL when a=b; otherwise it returns a.Great for turning zero into NULL before aggregation.

Example

SELECT NULLIF(stock,0) AS stock_or_null FROM Products;

Best practices for NULL handling

Prefer COALESCE for multiple fallbacks, avoid = NULL, and store defaults in schema when possible.

Can I filter aggregates that include NULL?

Yes. Use COUNT(column) to ignore NULLs or COUNT(*) to include them. Combine with WHERE column IS NOT NULL for clarity.

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Why How to Handle NULL Values in MySQL is important

How to Handle NULL Values in MySQL Example Usage


-- Report orders showing 0 when total_amount is NULL
SELECT o.id,
       COALESCE(o.total_amount, 0)        AS billed_amount,
       COALESCE(c.name, 'Guest')          AS customer_name
FROM Orders o
LEFT JOIN Customers c ON c.id = o.customer_id
WHERE o.order_date >= '2023-01-01'
  AND o.total_amount IS NOT NULL;

How to Handle NULL Values in MySQL Syntax


-- Detecting NULLs
SELECT * FROM Customers WHERE email IS NULL;

-- Replacing NULLs with defaults
SELECT id, IFNULL(email, 'unknown') AS email
FROM Customers;

-- Multiple fallbacks
SELECT id, COALESCE(total_amount, 0) AS total_amount
FROM Orders;

-- NULL-safe equality
SELECT * FROM Products WHERE price <=> NULL;

-- Converting a value to NULL
SELECT NULLIF(stock, 0) AS stock_or_null FROM Products;

Common Mistakes

Writing "WHERE column = NULL". This never matches because comparisons with NULL return NULL. Fix by using "WHERE column IS NULL".
Forgetting to wrap nullable columns in aggregates, causing NULL results. Use COALESCE to supply defaults before aggregating.

Frequently Asked Questions (FAQs)

Does COALESCE stop after the first non-NULL value?

Yes. MySQL evaluates arguments left to right and returns the first non-NULL, making it efficient for multiple fallbacks.

Is IFNULL faster than COALESCE?

IFNULL accepts exactly two arguments and has identical performance to COALESCE with two parameters. Choose based on readability.

Can indexes be used with IS NULL filters?

Yes. MySQL can use B-tree indexes to locate NULL values efficiently when the indexed column allows NULLs.

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