如何将datetime转换为datetime2？

Question

I am having an issue with a query currently where two fields in different tables are being used in a WHERE clause to filter data. Due to one being datetime and the other being datetime2, I am getting the

"Conversion failed when converting date and/or time from character string" error.

Here is the query:

/*| 1) SELECT - ABSENCES AND SCHEDULES |*/
------------------------------------------
    -- Retrieve data of absences
DECLARE @SADate AS DATETIME2(7)
SET @SADate = '2012-12-02'

    -- Retrieve absences that have a supply teacher and are within the current date.
SELECT SAStaffID
       ,LastName + ', ' + FirstName AS FullName
       ,SADate
       ,SSPeriod
       ,SSCourseCode
       ,SSRoom
    FROM tblOCStaffAbsent -- Join the staff schedule of the absent teacher that has a supply.
    INNER JOIN tblOCStaffSchedule S
        ON tblOCStaffAbsent.SAStaffID = S.SSStaffID
           AND @SADate BETWEEN S.SSStartDt AND S.SSEndDt
    INNER JOIN tblStaff SF
        ON SF.StaffID = SAStaffID
    INNER JOIN tblUsers U
        ON U.UserID = SF.StaffUserID
    WHERE SASupplyID IS NOT NULL
        AND SADate = @SADate

--------------------------------
/*| 2) SELECT - SUPERVISIONS |*/
--------------------------------
    --Union the supervisions
UNION ALL
SELECT SSupStaffID
       ,SupervisionDt
       ,CASE WHEN SSupPeriodHalf IS NOT NULL THEN SSupPeriod + '-' + SSupPeriodHalf
             ELSE SSupPeriod
        END AS FullPeriod
       ,'(SP) ' + SSupLocation
       ,NULL AS SSupRoom
       ,SSupMIDent
    FROM tblOCStaffSupervisions
    WHERE SupervisionDt = @SADate

SADate is the datetime field within tblOCStaffAbsent and SupervisionDt is the datetime2 field within tblOCStaffSupervisions.

Answer 1

The value for SupervisionDt which is coming from tblOCStaffSupervisions table may be of DateTime Datatype. The @SADate = '2012-12-02' is a string. @SADate should be set in DateTime Datatype.

Answer 2

If you look carefully at your error message, it is not problem of comparing DATETIME to DATETIME2 which can be done by sql server without any problems.

You have something stored as string but you are trying to convert it to DATETIME2 .

To convert to DATETIME2 is done like other converts CONVERT (DATETIME2, MyDatetimeField)

just know that DATETIME2 has higher precision than DATETIME . Your data will not change, only new information added with higher precision will use it.

Look at the following example. Both rows will produce same result.

DECLARE @Datetime DATETIME = '03/21/2014 15:23:34.444'

SELECT @Datetime
UNION ALL
SELECT CONVERT(DATETIME2, @Datetime)

but in the following case

DECLARE @datetime2 DATETIME2 = SYSDATETIME();

SELECT @datetime2
UNION ALL
SELECT CONVERT(DATETIME, @datetime2)

first select will produce higher precision point than second one