Nondeterministic conversion of literal date strings into DATE values
Use caution when allowing conversion of your CHARACTER strings into DATE data types. The reason is that such conversions are often nondeterministic.
SET LANGUAGE example: Month name in Polish
SET LANGUAGE Polish;
A character string can be the name of a month. But is the name in English, or Polish, or Croatian, or in another language? And, will the user's session be set to the correct corresponding LANGUAGE?
For example, consider the word listopad, which is the name of a month. But which month it is depends on the language the SQL system believes is being used:
- If Polish, then listopad translates to month 11 (November in English).
- If Croatian, then listopad translates to month 10 (October in English).
Code example of SET LANGUAGE
--SELECT alias FROM sys.syslanguages ORDER BY alias; DECLARE @yourInputDate NVARCHAR(32) = '28 listopad 2018'; SET LANGUAGE Polish; SELECT CONVERT(DATE, @yourInputDate) AS [SL_Polish]; SET LANGUAGE Croatian; SELECT CONVERT(DATE, @yourInputDate) AS [SL_Croatian]; SET LANGUAGE English; /*** Actual output: For the two months, note the 11 versus the 10. SL_Polish 2018-11-28 SL_Croatian 2018-10-28 ***/
SET DATEFORMAT example
SET DATEFORMAT dmy;
The preceding dmy format says that an example date string of '01-03-2018' would be interpreted to mean the first day of March in the year 2018.
If instead mdy was specified, then the same '01-03-2018' string would mean the third day of January in 2018.
And if ymd was specified, there is no guarantee of what the output would be. The numeric value of '2018' is too large to be a day.
In Japan and China, the DATEFORMAT of ymd is used. The format's parts are in a sensible sequence of largest unit to smallest. Consequently, this format sorts well. This format is considered to be the international format. It is international because the four digits of the year are unambiguous, and no country on Earth uses the archaic format of ydm.
In other countries such as Germany and France, the DATEFORMAT is dmy, meaning 'dd-mm-yyyy'. The dmy format does not sort well, but it is a sensible sequence of smallest unit to largest.
The United States, and the Federated States of Micronesia, are the only countries that use mdy, which does not sort. The format's mixed sequence matches a pattern of verbal speech in spoken dates.
Code example of SET DATEFORMAT: mdy versus dmy
The following Transact-SQL code example uses the same date character string with three different DATEFORMAT settings. A run of the code produces the output shown in the comment:
DECLARE @yourDateString NVARCHAR(10) = '12-09-2018'; PRINT @yourDateString + ' = the input.'; SET DATEFORMAT dmy; SELECT CONVERT(DATE, @yourDateString) AS [DMY-Interpretation-of-input-format]; SET DATEFORMAT mdy; SELECT CONVERT(DATE, @yourDateString) AS [MDY-Interpretation-of-input-format]; SET DATEFORMAT ymd; SELECT CONVERT(DATE, @yourDateString) AS [YMD-Interpretation--?--NotGuaranteed]; /*** Actual output: 12-09-2018 = the input. DMY-Interpretation-of-input-format 2018-09-12 MDY-Interpretation-of-input-format 2018-12-09 YMD-Interpretation--?--NotGuaranteed 2018-12-09 ***/
In the preceding code example, the final example has a mismatch between format ymd versus the input string. The third node of the input string represents a numeric value that is too large to be a day. Microsoft does not guarantee the output value from such mismatches.
CONVERT offers explicit codes for deterministic control of date formats
Our CAST and CONVERT documentation article lists explicit codes that can you can use with the CONVERT function to deterministically control date conversions. Every month the article has one of our highest pageview counts.
- CAST and CONVERT (Transact-SQL): Date and time styles
- CAST and CONVERT (Transact-SQL): Certain datetime conversions are nondeterministic
Compatibility level 90 and above
In SQL Server 2000, the compatibility level was 80. For level settings of 80 or below, implicit date conversions were deterministic.
Starting with SQL Server 2005 and its compatibility level of 90, implicit date conversions became nondeterministic. Date conversions became dependent on SET LANGUAGE and SET DATEFORMAT starting with level 90.
Conversion of non-Unicode character data between collations is also considered nondeterministic.
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