How to Handle NULL Values in SQL Server

How do I handle NULL values in SQL Server?

NULL handling in SQL Server lets you test, replace, and safely aggregate missing values so queries return the expected results.

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Description

Example H2

Example H3

Why do NULLs matter in SQL Server?

NULL represents “unknown.” Arithmetic, concatenation, and comparisons that involve NULL return NULL, which can break filters, joins, and reports.

How do I test for NULL values?

Use IS NULL or IS NOT NULL. Comparing with = NULL never works because NULL is not a value.

Syntax of IS NULL / IS NOT NULL

column_name IS [NOT] NULL in WHERE, JOIN, or CASE clauses.

How do I turn NULL into a default value?

Wrap the column in ISNULL() or COALESCE(). Both return the first non-NULL argument, letting you supply a fallback.

What is the difference between ISNULL and COALESCE?

ISNULL() is SQL Server–specific and returns the data type of its first argument; COALESCE() is ANSI and evaluates multiple inputs. Prefer COALESCE() for portability.

Can aggregates ignore NULLs?

Yes. SUM(), COUNT(col), AVG(), MIN(), and MAX() automatically skip NULL rows, so no extra handling is required.

How do I sort NULLs last?

Use a CASE expression in ORDER BY: ORDER BY CASE WHEN column IS NULL THEN 1 ELSE 0 END, column.

Best practices for NULL handling

Declare columns NOT NULL whenever possible, document when NULL is meaningful, and use COALESCE() in output layers instead of core tables.

Why How to Handle NULL Values in SQL Server is important

How to Handle NULL Values in SQL Server Example Usage


-- Show each customer's last order date; use 'N/A' when no orders exist
SELECT c.id,
       c.name,
       COALESCE(MAX(o.order_date)::varchar, 'N/A') AS last_order
FROM Customers c
LEFT JOIN Orders o ON o.customer_id = c.id
GROUP BY c.id, c.name;

How to Handle NULL Values in SQL Server Syntax


-- Test for NULL
SELECT *
FROM Orders
WHERE total_amount IS NULL;

-- Replace NULL with default
SELECT COALESCE(total_amount, 0) AS total_amount
FROM Orders;

-- Multiple fallbacks
SELECT COALESCE(Orders.total_amount, Products.price, 0) AS amount
FROM Orders
JOIN OrderItems ON Orders.id = OrderItems.order_id
JOIN Products    ON Products.id = OrderItems.product_id;

Common Mistakes

Mistake: Using = NULL in a WHERE clause. Why wrong: NULL is not equal to anything, so the filter returns zero rows. Fix: use IS NULL or IS NOT NULL.
Mistake: Relying on ISNULL for portability. Why wrong: ISNULL is SQL Server–specific and may change data types. Fix: use ANSI-standard COALESCE.