intro
Let’s learn everything you need to know about two of the most popular functions for handling NULL values in this ISNULL vs COALESCE comparison.
When it comes to handling NULL values in SQL, there is no shortage of options due to functions like NULLIF, ISNULL, COALESCE, etc. With so many choices, it is easy to get confused or unsure about which one is best for your specific use case. That is exactly where comparison guides like this ISNULL vs COALESCE post come in handy.
In this COALESCE vs ISNULL guide, you will dive into ISNULL and COALESCE to understand what they are, how they work, and when to use each one of them.
Time to dive in!
ISNULL vs COALESCE: What Are They and How Do They Work?
Let’s kick off this ISNULL vs COALESCE comparison blog post by looking at what these two NULL-handling functions are and how to use them.
ISNULL
Disclaimer: ISNULL is not part of the standard ANSI SQL. As a result, it is not available in all databases—for example, PostgreSQL does not support ISNULL. Even when it is supported, its behavior may vary slightly from one system to another.
In SQL Server, the ISNULL function replaces NULL values with the specified default value. This is the syntax to use it:
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ISNULL(expression, replacement)
Where
If expression is not NULL, ISNULL returns expression. If it is NULL, it returns replacement instead. As you might expect, this SQL Server function is commonly used to prevent unexpected behavior when dealing with missing data that has a sensible default (e.g., such as treating NULL as 0).
In MySQL, ISNULL works differently. It is used as a boolean function to test if a value is NULL, not to replace it. This is its syntax:
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ISNULL(expression)
And the following concerns the bevahior of ISNULL in MySQL:
Note: To replace NULL values in MySQL as in the SQL Server ISNULL function, you need to use the MySQL IFNULL function instead.
COALESCE
The SQL COALESCE function returns the first non-NULL value from a list of SQL expressions. Below is its syntax:
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COALESCE(expr_1, expr_2, ..., expr_N)
Where:
COALESCE evaluates each expression in order, returning the first one that is not NULL.
Note: COALESCE is ANSI-standard SQL and widely supported across databases like SQL Server, MySQL, Oracle, and PostgreSQL.
COALESCE vs ISNULL: Real-World Examples
The best way to understand the differences between COALESCE and ISNULL is by seeing practical examples.
Note: The sample queries below will be executed in DbVisualizer, a top-rated database client that supports over 50 databases. However, you can run them in any other multi-database client.
The table we will operate on is Users :
Time to see how to see ISNULL vs COALESCE null-handling approaches!
Null Checking
Use the MySQL ISNULL function to check if the user's addresses are NULL:
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SELECT
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name,
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main_address,
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secondary_address,
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ISNULL(main_address) AS is_main_null,
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ISNULL(secondary_address) AS is_secondary_null
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FROM users;
Assuming the SQL Server Users table is also available on a MySQL server, the result will be as follows:
You now see how NULL addresses become 1 values, while non-NULL addresses are converted into 0 values.
Simple NULL Replacement
Now, assume you want to get the user’s address, preferring main_address, but falling back to secondary_address if the main one is NULL.
With ISNULL in SQL Server, you can achieve that with the following SQL query:
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SELECT
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Name,
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ISNULL(MainAddress, SecondaryAddress) AS preferred_address
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FROM users;
The result will be as follows:
Note how Bob's preferred address falls back to their secondary address because their main address is NULL. Charlie, on the other hand, has no available address since both their main and secondary addresses are NULL. For all other users, the preferred address is simply the main address, as it is not NULL.
Keep in mind that you can achieve the same result using this COALESCE query:
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SELECT
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Name,
3
COALESCE(MainAddress, SecondaryAddress) AS preferred_address
4
FROM users;
COALESCE with two arguments behaves just like SQL Server’s ISNULL function.
Multi-Value NULL Replacement
Now, suppose you want to extend the previous example to display "N/A" when both addresses are NULL. Since this involves more than two fallback values, you'll need to use COALESCE:
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SELECT
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Name,
3
COALESCE(MainAddress, SecondaryAddress, 'N/A') AS preferred_address
4
FROM users;
The result will be:
As expected, Charlie now has "N/A" as the preferred address since both his addresses are NULL.
To achieve the same result using ISNULL, you would need to nest multiple function calls:
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SELECT
2
Name,
3
ISNULL(MainAddress, ISNULL(SecondaryAddress, 'N/A')) AS preferred_address
4
FROM users;
While this works, it is less elegant and makes the query harder to read and maintain compared to COALESCE.
ISNULL vs COALESCE: Comparison Table
For a quick comparison, take a look at the ISNULL vs COALESCE summary table below:
| Feature | ISNULL | COALESCE |
|---|---|---|
| Purpose | - Replaces NULL with a default value (in SQL Server)- Checks if a value is NULL (in MySQL) | Returns the first non-NULL value in a list |
| Arguments | Accepts exactly 2 arguments | Accepts 2 or more arguments |
| Data type resolution | Returns the data type of the first argument | Returns the data type with the highest precedence |
| ANSI SQL Standard | ❌ | ✅ |
| Portability | Low (limited support, varies by database and implementation) | High (widely supported across major databases) |
| Short-Circuit Evaluation | ❌ | ✅ Stops at the first non-NULL value |
| Available in MySQL | ✅ | ✅ |
| Available in PostgreSQL | ❌ | ✅ |
| Available in SQL Server | ✅ | ✅ |
| Available in Oracle | ❌ | ✅ |
| Typical Use Case | Simple NULL replacement in SQL Server or NULL checking in MySQL | Flexible multi-value, or standard-compliant fallback handling for NULL values |
Conclusion
In this SQL ISNULL vs COALESCE guide, you learned the key differences between these two NULL-handling functions, how to use them, and which scenarios they are best suited for.
Handling NULL values in your data becomes much easier with a tool that lets you visually explore your tables. That is exactly where a database client like DbVisualizer can help you!
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FAQ
What is the SQL COALESCE vs ISNULL difference?
The main difference is that ISNULL accepts exactly two arguments and is specific to certain databases like SQL Server or MySQL—with two very different implementations. Instead, COALESCE is an ANSI-standard function that accepts two or more arguments and returns the first non-NULL value. COALESCE is more portable across databases and supports multiple inputs, while ISNULL is simpler but less flexible and less widely supported.
What databases support the COALESCE function?
The COALESCE function is supported by most major relational databases, including SQL Server, PostgreSQL, MySQL and MariaDB, Oracle, SQLite, and many others.
What databases support the ISNULL function?
The ISNULL function is supported primarily by SQL Server. In MySQL, ISNULL exists but works as a NULL check, not for replacement. Most other databases like PostgreSQL and Oracle do not support ISNULL.
What is the SQL Server COALESCE vs ISNULL difference?
In SQL Server, both ISNULL and COALESCE replace NULL values, but they differ in flexibility and behavior. ISNULL accepts only two arguments and returns the data type of the first argument, while COALESCE accepts multiple arguments and returns the first non-NULL value. Note that in SQL Server, COALESCE behaves just like ISNULL when used with two arguments.
Why use a database client?
A graphical database client like DbVisualizer simplifies database management by enabling easy querying, administration, and data visualization. It offers an intuitive interface for handling tables, examining schema relationships, and building or debugging queries using its feature-rich SQL editor. Key features such as autocomplete, ER diagrams, and export tools make DbVisualizer a standout choice. Try the Pro version with a 21-day free trial!
