---
title: "Difference between COALESCE vs ISNULL."  
description: "Difference between COALESCE vs ISNULL."  
author: "ICSM Computer"  
published: 2026-05-07  
updated: 2026-05-27  
canonical: https://www.mindstick.com/forum/162078/difference-between-coalesce-vs-isnull  
category: "database"  
tags: ["database", "sql server"]  
reading_time: 4 minutes  

---

# Difference between COALESCE vs ISNULL.

**[Difference](https://www.mindstick.com/articles/157114/good-news-or-bad-news-and-the-difference-is) between [COALESCE](https://www.mindstick.com/forum/158915/explain-the-purpose-of-the-coalesce-function-in-sql-and-provide-an-example) vs ISNULL.**

## Replies

### Reply by ICSM Computer

Both `COALESCE` and `ISNULL` are used to handle `NULL` values in SQL, but they differ in behavior, portability, datatype handling, and standards compliance.

## 1. Basic Purpose

Both replace `NULL` with another value.

## Example

```plaintext
SELECT ISNULL(NULL, 'Default');
SELECT COALESCE(NULL, 'Default');
```

Output:

```plaintext
Default
```

## 2. Syntax Difference

## ISNULL

```plaintext
ISNULL(expression, replacement_value)
```

Accepts only 2 arguments

## COALESCE

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

Accepts multiple arguments

Returns first non-null value

## Example

## ISNULL

```plaintext
SELECT ISNULL(NULL, 10);
```

## COALESCE

```plaintext
SELECT COALESCE(NULL, NULL, 10, 20);
```

Output:

```plaintext
10
```

## 3. SQL Standard Support

| Feature | ISNULL | COALESCE |
| --- | --- | --- |
| ANSI SQL Standard | No | Yes |
| Database portability | Limited | Better |

## Database Support

## ISNULL

Mostly available in:

- Microsoft SQL Server

## COALESCE

Supported by almost all databases:

- Microsoft SQL Server
- MySQL
- PostgreSQL
- Oracle Database

## 4. Datatype Handling

This is one of the biggest differences.

## ISNULL Uses First Argument's Datatype

```plaintext
SELECT ISNULL(NULL, 'Hello');
```

Result datatype depends on first parameter.

## COALESCE Uses Highest Precedence Datatype

```plaintext
SELECT COALESCE(NULL, 1, 'Hello');
```

SQL determines datatype based on precedence rules.

## Example

```plaintext
SELECT ISNULL(NULL, 1.5);
```

May behave differently than:

```plaintext
SELECT COALESCE(NULL, 1.5);
```

because datatype resolution differs internally.

## 5. Evaluation Behavior

## ISNULL

- Evaluated once
- Simpler internal implementation

## COALESCE

Internally converted into a `CASE` expression.

Example:

```plaintext
COALESCE(a, b, c)
```

becomes:

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

This can sometimes evaluate expressions multiple times.

## 6. Performance

In most real-world cases:

- Difference is negligible
- Query optimization matters more

However:

- `ISNULL` can be slightly faster in SQL Server
- `COALESCE` is more flexible and portable

## 7. Nullability Behavior

This matters in computed columns and indexes.

## ISNULL

Often returns result as `NOT NULL`.

## COALESCE

May still be treated as nullable.

## Example

```plaintext
SELECT ISNULL(NULL, 0)
```

can be considered non-nullable.

But:

```plaintext
SELECT COALESCE(NULL, 0)
```

may still be treated differently by SQL Server metadata.

## Practical Examples

## Example Table

```plaintext
CREATE TABLE Employees
(
    Id INT,
    Bonus INT NULL
);
```

## Using ISNULL

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

## Using COALESCE

```plaintext
SELECT COALESCE(Bonus, 0) AS Bonus
FROM Employees;
```

Both replace `NULL` bonus values with `0`.

## Multiple Fallback Values

Only `COALESCE` supports this easily.

```plaintext
SELECT COALESCE(HomePhone, WorkPhone, MobilePhone, 'No Number')
FROM Contacts;
```

This returns the first available phone number.

## Key Differences Summary

| Feature | ISNULL | COALESCE |
| --- | --- | --- |
| Arguments supported | 2 | Multiple |
| ANSI standard | No | Yes |
| Portability | Low | High |
| Datatype returned | First argument type | Highest precedence type |
| Internal implementation | Function | CASE expression |
| SQL Server optimization | Slightly faster | Slightly flexible |
| Multiple fallback values | No | Yes |

## When to Use ISNULL

Use `ISNULL` when:

- Working only in SQL Server
- Need simple null replacement
- Want predictable datatype from first argument
- Slight performance optimization matters

## When to Use COALESCE

Use `COALESCE` when:

- Writing portable SQL
- Need multiple fallback values
- Following ANSI SQL standards
- Working across different databases

## Recommended Practice

For modern SQL development:

- Prefer `COALESCE` for portability and flexibility
- Use `ISNULL` when SQL Server-specific behavior is needed

## Final Example

```plaintext
SELECT
    Name,
    COALESCE(Nickname, FirstName, 'Unknown') AS DisplayName
FROM Users;
```

This creates a graceful fallback chain:

- Nickname
- FirstName
- "Unknown"


---

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