Date Conversion in SQL Server

Hello, Dev! Are you looking for a comprehensive guide to date conversion in SQL Server? Look no further! This article will cover everything you need to know, from converting date and time data types to working with different date formats.

Understanding Date and Time Data Types

Before we dive into date conversion, let’s take a moment to understand the different data types in SQL Server that relate to date and time:

Data Type	Description
DATE	Stores only the date, without any time information
TIME	Stores only the time, without any date information
DATETIME	Stores both date and time information
DATETIME2	Stores both date and time information, with higher precision than DATETIME
SMALLDATETIME	Stores both date and time information, but with a reduced range of dates
OFFSETDATETIME	Stores both date and time information, with an offset from UTC time

It’s important to note that when working with date and time data types, SQL Server uses the default format of ‘YYYY-MM-DD’ for dates and ‘HH:MI:SS.mmm’ for times.

Converting Between Date and Time Data Types

If you need to convert between different date and time data types, SQL Server provides several conversion functions:

CONVERT

The CONVERT function allows you to change the data type of an expression or value. For example, if you have a DATETIME value and want to convert it to a DATE value, you can use the following syntax:

CONVERT(DATE, datetimeValue)

The first parameter specifies the desired data type, and the second parameter is the value to be converted. Note that you can also use this function to specify the desired format for date and time values (more on this later).

CAST

The CAST function provides a similar functionality to CONVERT, but with a simpler syntax. For example, to convert a DATETIME value to a DATE value, you can use the following syntax:

CAST(datetimeValue AS DATE)

Note that CAST is a more efficient function than CONVERT, but it’s also more limited in terms of the available data types and formats.

SWITCHOFFSET

The SWITCHOFFSET function allows you to change the offset of an OFFSETDATETIME value. For example, if you have an OFFSETDATETIME value with an offset of -06:00 and want to change it to -05:00, you can use the following syntax:

SWITCHOFFSET(offsetDatetimeValue, '-05:00')

The first parameter is the value to be modified, and the second parameter is the desired offset.

Working with Different Date Formats

SQL Server provides several built-in date formats that you can use to display date and time information in different ways. These formats can be used with the CONVERT function (as mentioned earlier) to convert between different data types and formats.

Here are some of the most common date formats in SQL Server:

Format	Description
YYYY-MM-DD	Standard ISO format for dates
MM/DD/YYYY	US format for dates
DD/MM/YYYY	European format for dates
HH:MI:SS.mmm	Standard format for times

If none of these built-in formats meet your needs, you can also create custom formats using the FORMAT function. This function allows you to specify a format string that includes placeholders for different date and time components. For example, to display a date in the format ‘DD-Mon-YYYY’, you can use the following syntax:

FORMAT(dateValue, 'dd-MMM-yyyy')

For more information about custom format strings, see the official Microsoft documentation.

Frequently Asked Questions

Q: How do I convert a DATETIME value to a UNIX timestamp?

A: To convert a DATETIME value to a UNIX timestamp (i.e., the number of seconds since January 1, 1970), you can use the following formula:

DATEDIFF(SECOND, '1970-01-01', datetimeValue)

Q: How do I convert a DATE value to a DATETIME value?

A: To convert a DATE value to a DATETIME value, you can use the following syntax:

CAST(dateValue AS DATETIME)

This will add a default time value of ’00:00:00.000′ to the date value.

Q: How do I convert a DATETIME value to a specific timezone?

A: To convert a DATETIME value to a specific timezone, you can first convert it to an OFFSETDATETIME value using the TODATETIMEOFFSET function, and then use the SWITCHOFFSET function to change the offset. For example, to convert a DATETIME value to Central Standard Time (CST), you can use the following syntax:

SWITCHOFFSET(TODATETIMEOFFSET(datetimeValue, '-06:00'), '-05:00')

Q: How do I display dates in different languages?

A: To display dates in different languages, you can use the SET LANGUAGE command to switch to a different language setting. For example, to display dates in Spanish, you can use the following syntax:

SET LANGUAGE SpanishSELECT CONVERT(VARCHAR, dateValue, 101) AS SpanishDate

Note that the language setting only affects the display of dates, not the underlying data.

Q: How do I calculate the difference between two dates?

A: To calculate the difference between two dates, you can use the DATEDIFF function. For example, to calculate the number of days between two dates, you can use the following syntax:

DATEDIFF(DAY, dateValue1, dateValue2)

The first parameter specifies the unit of measurement (e.g., DAY, HOUR, SECOND), and the second and third parameters are the two dates to be compared.

Q: How do I add or subtract days from a date?

A: To add or subtract days from a date, you can use the DATEADD function. For example, to add 7 days to a date, you can use the following syntax:

DATEADD(DAY, 7, dateValue)

The first parameter specifies the unit of measurement (e.g., DAY, HOUR, SECOND), the second parameter is the number of units to add or subtract, and the third parameter is the date value to modify.

Conclusion

And there you have it, Dev! Everything you need to know about date conversion in SQL Server. We covered the different date and time data types, conversion functions, and date formats. We also provided answers to some common questions about working with dates in SQL Server. With this knowledge, you’ll be able to confidently convert and manipulate date and time values in your SQL code. Happy coding!