What is the difference between ISNULL() and COALESCE()?

Published on 17 Sep 2026 2 min read MS SQL

SQL Fundamentals

`ISNULL()` vs `COALESCE()`

Both functions replace NULL with another value.

Syntax

ISNULL(value, replacement)

COALESCE(value1, value2, ..., defaultValue)

Main differences

Feature	`ISNULL()`	`COALESCE()`
Standard	SQL Server-specific	ANSI SQL standard
Number of arguments	Exactly two	Two or more
Result	Returns replacement if first value is `NULL`	Returns the first non-`NULL` value
Return data type	Uses the first argument’s data type	Uses the data type with the highest precedence
Nullability metadata	Often treated as not nullable	May be treated as nullable
Evaluation	Function	Translated internally to a `CASE` expression

`ISNULL()` example

SELECT ISNULL(Bonus, 0) AS Bonus
FROM Employees;

If Bonus is NULL, it returns 0.

`COALESCE()` example

SELECT COALESCE(MobileNumber, HomeNumber, OfficeNumber, 'Not Available')
FROM Employees;

It returns the first non-NULL value.

Important data-type difference

ISNULL() normally uses the data type and length of its first argument.

DECLARE @Name VARCHAR(5) = NULL;

SELECT ISNULL(@Name, 'Mohamed');

Result:

Moham

The result is truncated because @Name is VARCHAR(5).

COALESCE() determines the result type based on SQL Server’s data-type precedence and expression rules:

DECLARE @Name VARCHAR(5) = NULL;

SELECT COALESCE(@Name, 'Mohamed');

Result:

Mohamed

In this example, the longer literal can determine the returned string length.

When to use

Use ISNULL() when:

You are working only with SQL Server.
You need to replace one value with one default value.
You want the first argument to control the result type.

ISNULL(Salary, 0)

Use COALESCE() when:

You need the first available value from multiple columns.
You want portable, standard SQL.
You understand its data-type precedence behaviour.

COALESCE(Email, MobileNumber, 'Not Available')

Interview answer

ISNULL() is SQL Server-specific and accepts exactly two arguments. COALESCE() is ANSI-standard SQL and accepts multiple arguments, returning the first non-NULL value. They can also differ in result data type and nullability.

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