Most efficient way in SQL Server to get date from date+time?

No magic, just efficiency: Understanding the CAST function

When dealing with large datasets, the efficiency of your SQL commands is crucial. The CAST function provides a straightforward way to convert the datetime to datewith minimal performance overhead. In SQL Server, it's like Rambo taking care of business: no fuss, just results.

For testing purposes or when you don't need to run the command on all rows, use a condition as shown:

SELECT CAST(YourDateTimeColumn AS date) AS DateOnly
FROM YourTable
WHERE SomeCondition = SomeValue;
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To analyze the performance, make your testing environment as close to the production environment as possible.

Digging deeper: Alternative methods & performance tips

SQL Server offers you a variety of tools to extract the date. It's time to explore these alternatives:

Using CONVERT function

CONVERT works similarly to CAST, but offers style flexibility:

SELECT CONVERT(date, YourDateTimeColumn) AS DateOnly FROM YourTable;
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This is like asking King Arthur to deliver your pizza. Overkill, but gets the job done!

Date manipulation with DATEADD and DATEDIFF

These functions come in handy for date arithmetic or to get the start of the day:

SELECT DATEADD(DAY, DATEDIFF(DAY, 0, YourDateTimeColumn), 0) FROM YourTable;
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It's like finding a cassette player for your vintage mixtape collection. Tricky but still works!

Testing performance: The top tip

Run your queries on limited sets for testing efficiency using SELECT TOP:

SELECT TOP 1000000 CAST(YourDateTimeColumn AS date) FROM YourTable;
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SQL Server will thank you for not overstressing the system CPU!

SARGability and more

When writing queries, ensure they are sargable i.e., SQL Server can efficiently use indexes. Both CAST and CONVERT are sargable operations.

Pitfalls to avoid

Converting datetime to date may be straightforward, but beware of edge cases. For example, with datetime2, a high-precision data type, timestamp precision can be lost.

Simplicity for performance

For data processing and reporting, keeping it simple is the key. Using built-in functions like CAST and CONVERT, you leverage SQL Server's processing power, minimizing impact on performance while keeping your code clean and readable.