Understanding T-SQL DateTime Conversion Behavior

When working with dates and times in Microsoft SQL Server, it’s essential to understand the behavior of date and time data types, including datetime, decimal, and float. In this article, we’ll delve into a specific issue related to converting decimals and floats back to datetime values.

What’s Happening?

The problem arises when converting a datetime value to decimal or float format using the CAST() function, and then attempting to convert that decimal or float value back to datetime using SELECT CAST(...) AS datetime. The issue seems to be related to how these data types are stored internally in SQL Server.

Understanding Data Types

In T-SQL, the following data types are relevant:

• datetime: a 7-byte integer representing a date and time from January 1, 1753, through December 31, 9999.
• decimal (with scale): a user-defined numeric type for precision and scale values between 0-28, 29-38, or 39-50. When used in CAST(), the scale is determined by the decimal places specified.
• float: a floating-point number representing a real value.

Decimal Conversion Behavior

When converting a datetime value to decimal format using CAST(GETDATE() AS decimal), SQL Server uses the internal representation of the datetime value as an integer, taking into account the precision and scale. This conversion can result in a loss of time precision, especially when dealing with long dates or times.

SELECT [date] = GETDATE()

, 
  -- When converting to decimal format, precision is limited by the scale.
  CAST(GETDATE() AS decimal) 

In this example, GETDATE() returns a datetime value. The CAST() function converts this value to decimal format, where the precision depends on the specified scale.

Float Conversion Behavior

Similarly, when converting a datetime value to float format using CAST(GETDATE() AS float), SQL Server’s behavior is identical to its decimal conversion process.

SELECT [date] = GETDATE()

, 
  -- When converting to float format, precision depends on the system's precision setting.
  CAST(GETDATE() AS float)

The float data type in T-SQL uses a fixed-point representation with a limited number of significant digits. This means that when storing or comparing decimal values, the loss of precision can occur due to rounding errors.

datetime Conversion from Decimal and Float

When converting a decimal or float value back to datetime format using SELECT CAST(...) AS datetime, SQL Server interprets the value as an integer before attempting to represent it as a date and time. This process involves calculations that may result in unexpected outcomes, especially when dealing with dates that span multiple days.

DECLARE @dt datetime = '2009-08-15'

-- When converting from decimal format:
SELECT CAST(@dt AS decimal) -- 40038

DECLARE @dec1 decimal = 40038;
SELECT CAST(@dec1 AS datetime) -- 2009-08-15 00:00:00.000

datetime Conversion from Float

When converting a float value to datetime format using SELECT CAST(...) AS datetime, SQL Server’s behavior is similar, with calculations leading to unexpected outcomes:

DECLARE @dt datetime = '2009-08-15'

-- When converting from float format:
SELECT CAST(@dt AS float) -- 40038

DECLARE @flo1 float = 40038;
SELECT CAST(@flo1 AS datetime) -- 2009-08-15 00:00:00.000

Additional Considerations and Recommendations

• Use datetime2 data type: When working with dates and times, use the datetime2 data type instead of datetime. The datetime2 data type allows for a broader range of precision (100 nanoseconds) compared to the original datetime data type.
• Avoid implicit conversion: In your code, avoid implicitly converting between different data types. Instead, use explicit casting using CAST() or CONVERT().
• Validate user input: When storing and retrieving datetime values from user input sources, always validate the entered data to prevent potential issues.

DECLARE @dt datetime2 = '2009-08-15'

-- Validate and store user input:
IF (ISDATE(@input) = 1)
BEGIN
    SET @dt = CAST(@input AS datetime2);
END

-- Convert from decimal or float format using explicit casting:

SELECT [date] = GETDATE()

, 
  -- When converting to decimal format:
  CAST(GETDATE() AS decimal)

DECLARE @dec1 decimal = 40038;
SELECT CAST(@dec1 AS datetime2) -- 2009-08-15 00:00:00.000

Conclusion

When working with dates and times in Microsoft SQL Server, it’s crucial to understand how different data types are represented internally. By being aware of these differences and following best practices, you can avoid common pitfalls and create more reliable applications.

In this article, we discussed the conversion behavior between datetime, decimal, and float data types, including implicit conversions that may result in precision loss. We also highlighted the importance of using explicit casting, validating user input, and choosing suitable data types to ensure accurate results.

Last modified on 2024-02-17