How do you update a DateTime field in T-SQL?

Best practices in updating DateTime fields

The path to an efficient UPDATE statement involves several best practices:

DateTime formatting consistency

Use the ISO 8601 YYYY-MM-DDThh:mm:ss format to prevent misinterpretation.

-- Like a boss, you've mastered ISO 8601 format
UPDATE ProjectTable
SET StartDate = '2023-03-14T09:00:00'
WHERE ProjectID = 101; 


WHERE clause accuracy

Ensure the WHERE clause targets the correct records. Remember, with great power, comes great responsibility - use wisely!

Update testing

Conducting update tests in a controlled environment prior to live deployment can save you from migraines down the line.

Keep data types compatible

The WHERE condition should have compatible data types for the comparison. For instance, if your 'Id' column is numeric, avoid wrapping the comparison value in single quotes.

Tips for robust updates

A few additional pointers to build robust UPDATE strategies:

Be aware of regional settings

The YYYY-MM-DD format may trip over language settings. But fear not! The YYYYMMDD format is your friend here.

Debug, debug, debug!

Debug if an update didn't go as planned. Check your table/column names, validate data types, and beware of added characters messing with format.

Post-update record check

After executing the UPDATE statement, confirm the changes using a SELECT command. This step ensures that the UPDATE statement has done its job.

Handle date formatting issues

If date formatting gives you trouble, put your trust in Microsoft's comprehensive guide on T-SQL data type conversion.

When millisecond precision counts

Handling milliseconds in DateTime

For update operations that require millisecond precision, go for 'YYYY-MM-DD hh:mm:ss.fff'

-- One does not simply ignore milliseconds! 
UPDATE TimeSensitiveTable 
SET PreciseTimeColumn = '2023-03-14 15:45:00.123' -- Splitting seconds, literally!
WHERE YourCondition;


SQL functions to the rescue

Use SQL Server's built-in functions like DATEADD() and DATEDIFF() for updates that need to manipulate DateTime values.

UPDATE EventTable
SET EventDateTime = DATEADD(hour, 2, EventDateTime) -- Adds two hours because why not?
WHERE EventDateTime < GETDATE();


Dealing with edge cases in DateTime updates

Handling NULL values

For NULL values in DateTime columns, establish a strategy. Do you set a default time, ignore these records, or use GETDATE() to fill with current time?

DateTime updates across multiple rows

If you need to update dates incrementally across rows, turn to window functions like ROW_NUMBER(). Combine with DATEADD() to give each record a unique DateTime.

TimeZone aware updates

For systems operating across timezones, consider the conversion between local time and UTC. Use AT TIME ZONE when updating DateTimeOffset columns.