When working with numbers in SQL Server, both ROUND() and FORMAT() can be used to round the numeric values to a given decimal precision. But that doesn’t mean we should use them interchangeably. Each function serves a different purpose and has its distinct use cases.
ROUND() Function
ROUND() is primarily a mathematical function that changes the actual numeric value. It allows us to round a number to a specified length or precision:
SELECT
ROUND(123.456, 2) AS n1,
ROUND(123.454, 2) AS n2;Output:
n1 n2
------ ------
123.46 123.45
The ROUND() function returns a numeric value:
SELECT SQL_VARIANT_PROPERTY(ROUND(123.456, 2), 'BaseType');Output:
numeric
Main characteristics of the ROUND() function:
- Returns a numeric data type (preserves the original data type)
- Actually changes the mathematical value
- Faster performance since it’s a pure mathematical operation
- Supports negative precision for rounding to tens, hundreds, etc.
- Can specify rounding behavior (round up/down) with optional third parameter
FORMAT() Function
FORMAT() is a presentation function that converts values to formatted strings. So the output is a string even if we pass a number. Here’s an example of this function:
SELECT
FORMAT(123.456, '0.00') AS n1,
FORMAT(123.454, '0.00') AS n2;Output:
n1 n2
------ ------
123.46 123.45
The result might look the same as with the ROUND() function, but it actually returns a string:
SELECT SQL_VARIANT_PROPERTY(FORMAT(123.456, '0.00'), 'BaseType');Output:
nvarchar
Main characteristics of the FORMAT() function:
- Always returns a string (
NVARCHAR) - Designed for display purposes
- More flexible formatting options (currency, percentages, custom patterns)
- Slower performance due to string conversion and formatting
- Culture-aware formatting capabilities
When to Use Each
Here’s a quick guide to help determine which function to use for a given scenario:
Use ROUND() when: | Use FORMAT() when: |
|---|---|
| You need to perform further mathematical calculations | Preparing data for display to end users |
| Working with data that will be stored or used in computations | Generating reports or user interfaces |
| Performance is critical | Need specific formatting like currency symbols or thousand separators |
| You want to maintain numeric data types | Working with different cultural number formats |
| Implementing business logic that requires specific precision | Final output step in a query chain |
Performance Considerations
As mentioned, ROUND() is significantly faster than FORMAT(). This is because it performs simple mathematical operations, whereas FORMAT() involves string conversion and pattern matching. For large datasets or frequently executed queries, ROUND() is the better choice for performance.
Best Practice
Use ROUND() for data processing and mathematical operations, then apply FORMAT() only at the presentation layer when displaying results to users (and only if required). This approach maintains data integrity while providing the flexibility needed for user-friendly output.