CAST vs. DATETRUNC: Choosing the Right Method to Truncate Dates in SQL Server

1. Using CAST: This method is simple and works for all SQL Server versions. You cast the datetime value to the date data type. For example:

SELECT CAST(GETDATE() AS date) AS TruncatedDate;

This will return the current date without the time.

1. Using DATETRUNC function (preferred): This function offers more flexibility and is generally recommended. It allows you to truncate the datetime to various intervals like year, month, day, hour, minute, or even second. Here's the syntax:

DATETRUNC(truncation_part, datetime_expression)

• truncation_part: Specifies the part to truncate to (e.g., 'year', 'month', 'hour').
• datetime_expression: The datetime value you want to truncate.

For example, to truncate to the start of the day:

SELECT DATETRUNC('day', GETDATE()) AS Today;

This will return the current date at midnight (00:00:00).

Choosing the right method:

• Use CAST if you simply need the date part.
• Use DATETRUNC for more granular control over truncation (year, month, hour, etc.).

Additional points:

• Both methods are efficient and recommended over converting datetime to string and manipulating it.
• DATETRUNC is available in SQL Server 2012 and later versions.

DECLARE @myDateTime datetime = '2024-04-12 15:39:00';

-- Truncate to date using CAST
SELECT 'CAST:', CAST(@myDateTime AS date) AS TruncatedDate;

This code declares a variable @myDateTime with a specific datetime value. It then uses CAST to convert it to the date data type, effectively truncating the time portion.

Truncating to Different Parts with DATETRUNC:

DECLARE @myDateTime datetime = '2024-04-12 15:39:00';

-- Truncate to year
SELECT 'Year:', DATETRUNC('year', @myDateTime) AS TruncatedYear;

-- Truncate to month
SELECT 'Month:', DATETRUNC('month', @myDateTime) AS TruncatedMonth;

-- Truncate to hour
SELECT 'Hour:', DATETRUNC('hour', @myDateTime) AS TruncatedHour;

-- Truncate to minute (seconds and milliseconds are also supported)
SELECT 'Minute:', DATETRUNC('minute', @myDateTime) AS TruncatedMinute;

This code showcases DATETRUNC's flexibility. It truncates the same datetime value to different parts (year, month, hour, minute) and displays the results with labels.

Truncating to Today (Start of Day):

SELECT 'Today:', DATETRUNC('day', GETDATE()) AS Today;

This method relies on an internal detail of how SQL Server stores datetime values. It's generally not recommended due to its dependence on implementation specifics and potential lack of portability across different SQL Server versions. Here's how it works:

DECLARE @myDateTime datetime = '2024-04-12 15:39:00';

-- Convert datetime to float (implementation specific)
SELECT 'CAST to Float:', FLOOR(CAST(@myDateTime AS float)) AS TruncatedDate;

• Explanation:
  • We cast the datetime value to float. This utilizes an internal representation where the time portion is stored in the decimal part.
  • We then use the FLOOR function to remove the decimal portion, effectively truncating the time.
  • Finally, we cast the result back to datetime.

Important Considerations:

• This method relies on an undocumented implementation detail and might not work consistently across different SQL Server versions or platforms.
• It's less readable and can be confusing for developers unfamiliar with the internal workings.