Truncating Decimals in SQL Server

Understanding Truncation:

• Definition: Truncation is the process of cutting off digits after a specified decimal point, without considering the remaining digits for rounding purposes.
• Contrast with Rounding: Unlike rounding, which adjusts the last retained digit based on the following digits, truncation simply discards them.

SQL Server Functions:

• TRUNCATE() function: This is the most direct method for truncation in SQL Server. It takes two arguments:
  • The numeric value to be truncated.
  • The desired number of decimal places.
  • Example: TRUNCATE(123.45678, 2) will result in 123.45.
• CAST or CONVERT functions: These functions can also be used for truncation by specifying a data type with the desired number of decimal places. For example, CAST(123.45678 AS DECIMAL(10,2)) will truncate to 123.45.

Practical Applications:

• Data Cleanup: Truncation can be used to remove unwanted decimal places from data that is too precise for a specific application.
• Financial Calculations: In some cases, financial calculations may require truncation rather than rounding to avoid introducing errors due to rounding.
• Data Presentation: Truncation can be used to format data for display purposes, such as displaying monetary values with a fixed number of decimal places.

Example:

DECLARE @value DECIMAL(10, 5) = 123.45678;

-- Using TRUNCATE function
SELECT TRUNCATE(@value, 2) AS TruncatedValue;

-- Using CAST function
SELECT CAST(@value AS DECIMAL(10, 2)) AS TruncatedValue;

Key Points:

• Truncation is a straightforward process of cutting off digits after a specified decimal point.
• SQL Server provides the TRUNCATE() function and the CAST or CONVERT functions for truncation.
• Truncation can be useful for data cleanup, financial calculations, and data presentation.

Understanding the Example Codes

Example 1: Using the TRUNCATE() function

DECLARE @value DECIMAL(10, 5) = 123.45678;

SELECT TRUNCATE(@value, 2) AS TruncatedValue;

• Explanation:
  • DECLARE @value DECIMAL(10, 5) = 123.45678; declares a variable named @value of type DECIMAL with 10 total digits and 5 decimal places, and assigns it the value 123.45678.
  • SELECT TRUNCATE(@value, 2) AS TruncatedValue; selects the truncated value of @value using the TRUNCATE() function. The second argument, 2, specifies that we want to truncate the value to 2 decimal places. The result will be 123.45.

DECLARE @value DECIMAL(10, 5) = 123.45678;

SELECT CAST(@value AS DECIMAL(10, 2)) AS TruncatedValue;

• Explanation:
  • DECLARE @value DECIMAL(10, 5) = 123.45678; is the same as in Example 1.

• Both examples achieve the same result: truncating the decimal value to 2 decimal places.
• The TRUNCATE() function is specifically designed for truncation, while the CAST function is more general-purpose but can be used for truncation by specifying a data type with the desired number of decimal places.
• The choice between TRUNCATE() and CAST() is often a matter of personal preference or coding style.

Alternative Methods for Truncating Decimals in SQL Server

While the TRUNCATE() and CAST functions are the most common methods for truncating decimals in SQL Server, there are a few other approaches that you can consider:

Using String Functions:

• Convert to string and substring: Convert the decimal value to a string, extract the desired portion using SUBSTRING, and then convert it back to a decimal.

DECLARE @value DECIMAL(10, 5) = 123.45678;

SELECT CAST(SUBSTRING(CAST(@value AS VARCHAR(20)), 1, CHARINDEX('.', CAST(@value AS VARCHAR(20)))+2) AS DECIMAL(10,2)) AS TruncatedValue;

This method can be more verbose and less efficient than using TRUNCATE() or CAST.

Mathematical Operations:

• Multiply and divide: Multiply the decimal value by a power of 10 to shift the decimal point, truncate the integer part, and then divide by the same power of 10 to restore the original decimal point.

DECLARE @value DECIMAL(10, 5) = 123.45678;

SELECT TRUNCATE(@value * 100, 0) / 100 AS TruncatedValue;

Custom Scalar Functions:

• Create a user-defined scalar function: Define a custom scalar function that encapsulates the truncation logic.

CREATE FUNCTION dbo.TruncateDecimals (@value DECIMAL(10, 5), @decimals INT)
RETURNS DECIMAL(10, 5)
AS
BEGIN
    RETURN CAST(@value AS DECIMAL(10, @decimals));
END
GO

SELECT dbo.TruncateDecimals(123.45678, 2) AS TruncatedValue;

This method can provide more flexibility and reusability, but it requires additional development effort.