Wednesday, March 18, 2015

T-SQL: Inconsistent behavior between parameters and XML value

Recently I created a framework for translating JSON requests from a REST API to entities sent to the database. For simple objects, it was very easy, just create an SQL parameter for each property. However, for complex objects - having other objects as properties - this was not a solution. So I used a DataContractSerializer to transform the object to XML, send it as an XML SQL parameter and get the values from it in the stored procedures. Then I noticed date time inconsistencies between the two approaches. What was going on?

Let's start with the code. The DateTime object created from the JSON is a date and time value with a timezone, like 16:00 UTC+1. That is 15:00 in universal time. One you send it as a parameter for a stored procedure, the value received by the stored procedure is 16:00 (the server has the same timezone). In SQL Server, DATETIME and DATETIME2 types don't store timezone information. However, when sent through XML, the value looks like this: 2015-03-09T16:00:0.0000000+01:00. Using SELECT [Time] = T.Item.value('@Time','DATETIME2') FROM @Xml.nodes('//Location/SDO') as T(Item), the value returned is 15:00! You get 16:00+01 if you translate to DATETIMEOFFSET.

So let's recap: When you send a DateTime with timezone offset as an SQL parameter, the value reaching the SQL server is the local time. When you extract a textual value with timezone offset from an XML into a DATETIME, using the .value method, the value you get back is basically the Universal Time.

Solutions? Well, if you are going to use DateTime, you might as well consider that servers and devices are not always in the same timezone. Always translating values to universal time might be a good idea. Another solution is to extract from XML to a DATETIMEOFFSET type, which holds both the datetime and the timezone offset. Converting that value to DATETIME or DATETIME2 removes the timezone (Warning: it does NOT give the local time, unless you are in the same zone as the timezone in the datetimeoffset value).