What is difference between COALESCE vs ISNULL in SQ Server?

In Microsoft SQL Server, both COALESCE and ISNULL are used to handle NULL values, but they differ in behavior, standards compliance, datatype handling, and flexibility.

1. Basic Syntax

ISNULL

ISNULL(expression, replacement_value)

Example:

SELECT ISNULL(NULL, 'N/A');

Result:

N/A

COALESCE

COALESCE(value1, value2, value3, ...)

Example:

SELECT COALESCE(NULL, NULL, 'N/A');

Result:

N/A

2. Main Differences

3. Number of Arguments

ISNULL → Only 2 values

ISNULL(value, replacement)

COALESCE → Multiple values

COALESCE(value1, value2, value3, ...)

Example:

SELECT COALESCE(NULL, NULL, NULL, 'Default');

Very useful when checking several columns.

4. Datatype Handling (Important Difference)

ISNULL returns datatype of FIRST argument

Example:

SELECT ISNULL(NULL, 1.5);

SQL Server may infer datatype unexpectedly depending on first argument.

Another example:

SELECT ISNULL(CAST(NULL AS VARCHAR(3)), 'ABCDEFG');

Result:

ABC

Because:

• first argument datatype = VARCHAR(3)
• replacement gets truncated

COALESCE uses datatype precedence

Example:

SELECT COALESCE(CAST(NULL AS VARCHAR(3)), 'ABCDEFG');

Result:

ABCDEFG

This behavior is often safer and more intuitive.

5. ANSI Standard Compliance

COALESCE

ANSI SQL standard

Works across many databases:

• SQL Server
• PostgreSQL
• Oracle
• MySQL

ISNULL

• SQL Server-specific
• Not portable

6. Internal Behavior

COALESCE is internally translated into a CASE expression.

Equivalent:

COALESCE(a, b, c)

becomes roughly:

CASE
    WHEN a IS NOT NULL THEN a
    WHEN b IS NOT NULL THEN b
    ELSE c
END

7. Performance Difference

Usually:

• negligible difference
• both are fast in normal usage

Historically:

• ISNULL could be slightly faster in very specific cases
• modern SQL Server optimization makes this mostly irrelevant

8. When to Use Which?

Use ISNULL when:

• You only need 2 arguments
• You want SQL Server-specific behavior
• You intentionally want first argument datatype

Example:

SELECT ISNULL(MiddleName, 'Not Provided')
FROM Employees;

Use COALESCE when:

• You need multiple fallback values
• You want ANSI-standard SQL
• You want better portability
• You want datatype precedence handling

Example:

SELECT COALESCE(HomePhone, WorkPhone, MobilePhone, 'No Contact')
FROM Customers;

9. Common Interview Question

Which is better?

Most developers prefer:

• COALESCE for portability and flexibility
• ISNULL for simple SQL Server-only replacements

A practical rule:

Use COALESCE for standard, flexible SQL.
Use ISNULL for quick SQL Server-specific null replacement.

Quick Summary