SQL COALESCE vs ISNULL: Key Differences

By Sharon Ben-Moshe · August 26, 2026 · 2 min read

Sharon Ben-Moshe is the founder of sqlfmt, a browser-based SQL formatter and validator with misspelled-keyword suggestions across PostgreSQL, MySQL, SQLite, SQL Server, and BigQuery.

Compare SQL COALESCE and ISNULL: argument count, portability, return types, nullability, evaluation behavior, and the MySQL naming trap.

Quick answer

Use COALESCE when SQL must work across databases or when more than two fallback values are needed. In SQL Server, ISNULL takes two arguments and follows different type and nullability rules; in MySQL, ISNULL(expr) is a null test rather than a replacement function.

FAQs

Is COALESCE standard SQL?

COALESCE is a SQL-standard conditional expression and is broadly available across major SQL databases. It returns the first non-NULL argument, making it the safer default when a query needs to remain portable.

Is SQL Server ISNULL the same as COALESCE?

No. SQL Server ISNULL accepts two arguments and uses the first argument’s data type. COALESCE accepts a variable number of arguments and uses CASE-expression type-precedence rules; the expressions can also have different nullability metadata.

Can I use ISNULL to replace NULL in MySQL?

Not with SQL Server’s meaning. In MySQL, ISNULL(expr) tests whether expr is NULL. Use COALESCE(expr, fallback) or IFNULL(expr, fallback) to return a replacement value.

Should I use COALESCE or IS NULL?

Use COALESCE to return a fallback value. Use IS NULL in a predicate when filtering or branching on missing data. They solve different problems, and equality written as = NULL does not test for nulls correctly.

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Key takeaways

COALESCE returns the first non-NULL expression and accepts two or more expressions.

SQL Server ISNULL accepts two arguments and uses the first argument’s data type, unlike COALESCE’s type-precedence rules.

MySQL ISNULL(expr) answers whether expr is NULL; use COALESCE or IFNULL for a replacement value in MySQL.

How does SQL Server ISNULL differ from COALESCE?

Microsoft’s SQL Server documentation explains that COALESCE and ISNULL differ in argument count, evaluation, data-type selection, and nullability. ISNULL is evaluated once and uses the first parameter’s type; COALESCE can accept more arguments and follows CASE type-precedence rules.

Make the desired result type explicit

SELECT COALESCE(CAST(short_code AS varchar(20)), 'unknown') AS code
FROM products;

An explicit CAST is often the clearest choice when a fallback literal could otherwise be converted or truncated under dialect-specific rules.

What does ISNULL mean in MySQL?

MySQL uses ISNULL(expr) as a null predicate that returns whether its argument is NULL. That name is easy to carry over incorrectly from SQL Server; use COALESCE(expr, fallback) or MySQL IFNULL(expr, fallback) when the goal is a replacement value.

Use a portable fallback in MySQL

SELECT COALESCE(phone_number, 'not provided') AS phone_number
FROM customers;

When should you use COALESCE?

Choose COALESCE for portable retrieval logic and multiple fallback values. Use a dialect-specific function only when its documented type, nullability, or evaluation behavior is required and the query is intentionally tied to that database.

For the related rule that tests rather than replaces missing values, see why = NULL never works in SQL.

Format null-handling expressions before review with sqlfmt so the fallback order is visible instead of hidden in a long SELECT list.

SQL COALESCE vs ISNULL: Key Differences

Quick answer

FAQs

Is COALESCE standard SQL?

Is SQL Server ISNULL the same as COALESCE?

Can I use ISNULL to replace NULL in MySQL?

Should I use COALESCE or IS NULL?

The Comprehensive Guide to SQL Data Types: Storage, Precision, and Cross-Dialect Portability

The Definitive Guide to SQL Window Functions: Syntax, Framing, and Real-World Recipes

The Ultimate Guide to SQL JOINs: Visualizing Every Join Type, Cardinality, and Performance

Key takeaways

What does COALESCE do in SQL?

How does SQL Server ISNULL differ from COALESCE?

What does ISNULL mean in MySQL?

When should you use COALESCE?

Quick answer

FAQs

Is COALESCE standard SQL?

Is SQL Server ISNULL the same as COALESCE?

Can I use ISNULL to replace NULL in MySQL?

Should I use COALESCE or IS NULL?

Keep reading

The Comprehensive Guide to SQL Data Types: Storage, Precision, and Cross-Dialect Portability

The Definitive Guide to SQL Window Functions: Syntax, Framing, and Real-World Recipes

The Ultimate Guide to SQL JOINs: Visualizing Every Join Type, Cardinality, and Performance

Key takeaways

What does COALESCE do in SQL?

How does SQL Server ISNULL differ from COALESCE?

What does ISNULL mean in MySQL?

When should you use COALESCE?