我們應該在比較時轉換xml的左側操作數嗎？

Question

此鏈接https://docs.microsoft.com/en-us/sql/t-sql/xml/exist-method-xml-data-type有這樣的代碼：

declare @x xml; 
declare @f bit; 

set @x = '<root Somedate = "2002-01-01Z"/>'; 
set @f = @x.exist('/root[(@Somedate cast as xs:date?) eq xs:date("2002-01-01Z")]'); 

select @f; 

上面的鏈接說

相反鑄爲XS：日期（），您可以使用XS：日期（）構造函數。

因此，我已經制定如下，請確認上面的線意味着我已經推斷的相同。

這不是使用強制轉換爲xs：date。

declare @x xml; 
declare @f bit; 

set @x = '<root Somedate = "2002-01-01Z"/>'; 
set @f = @x.exist('/root[xs:date(@Somedate) eq xs:date("2002-01-01")]'); 

select @f; 

此致

Answer 1

我必須承認，我有點納悶這些結果...

顯然XQuery中存在的DATE和DATETIME更近的解釋，那麼我預計這一點。特別是列Equals4和Equals5從來都不是真的...

我所學到的是，當你想比較日期或dateTime值時，一個必須非常carefull ...一些演員將打破，因爲有顯然日期時間的隱式截斷......

試試這個：

declare @x xml; 

--Somedate1 is your Zone-format 
--Somedate2 is a simple ISO8601 date 
--Somedate3 is a simple ISO8601 dateTime 

set @x = '<root Somedate1 = "2002-01-01Z" 
       Somedate2 = "2002-01-01" 
       Somedate3 = "2002-01-01T00:00:00"/>'; 

--Compare it with a variable with the date as string (try without the "Z" too) 
--and a variable with a "real" DATETIME (try with DATE too => Equals9 will not work anymore) 
declare @DtAsString VARCHAR(100)='2002-01-01Z'; 
declare @DtAsDateTime DATETIME={d'2002-01-01'}; 

SELECT 
     SimpleData = @x.query('data(/root/@Somedate1)[1]') 
     ,DateCtor = @x.query('xs:date((/root/@Somedate1)[1])') 
     ,DtTmCtor = @x.query('xs:dateTime((/root/@Somedate1)[1])') 
     ,CastToDate = @x.query('(/root/@Somedate1)[1] cast as xs:date?') 
     ,ToDateTime = @x.query('(/root/@Somedate1)[1] cast as xs:dateTime?') 
     ,Val2Date = @x.value('(/root/@Somedate1)[1]','date') 
     ,Val2DtTime = @x.value('(/root/@Somedate1)[1]','datetime') 
     ,Equals1 = @x.exist('/root[@Somedate1 eq sql:variable("@DtAsString")]') 
     ,Equals2 = @x.exist('/root[(@Somedate1) eq xs:date(sql:variable("@DtAsString"))]') 
     ,Equals3 = @x.exist('/root[xs:date(@Somedate1) eq xs:date(sql:variable("@DtAsString"))]') 

     ,Equals4 = @x.exist('/root[xs:dateTime(@Somedate1) eq xs:dateTime(sql:variable("@DtAsString"))]') 
     ,Equals5 = @x.exist('/root[@Somedate1 cast as xs:dateTime? eq xs:dateTime(sql:variable("@DtAsString"))]') 
     ,Equals6 = @x.exist('/root[xs:date(@Somedate1) eq xs:date(sql:variable("@DtAsString"))]') 
     ,Equals7 = @x.exist('/root[@Somedate1 cast as xs:date? eq xs:date(sql:variable("@DtAsString"))]') 

     ,Equals8 = @x.exist('/root[@Somedate1 cast as xs:date? eq sql:variable("@DtAsString") cast as xs:date?]') 
     ,Equals9 = @x.exist('/root[@Somedate1 cast as xs:dateTime? eq sql:variable("@DtAsDateTime") cast as xs:dateTime?]') 

UNION ALL SELECT 
     @x.query('data(/root/@Somedate2)[1]') 
     ,@x.query('xs:date((/root/@Somedate2)[1])') 
     ,@x.query('xs:dateTime((/root/@Somedate2)[1])') 
     ,@x.query('(/root/@Somedate2)[1] cast as xs:date?') 
     ,@x.query('(/root/@Somedate2)[1] cast as xs:dateTime?') 
     ,@x.value('(/root/@Somedate2)[1]','date') 
     ,@x.value('(/root/@Somedate2)[1]','datetime') 
     ,@x.exist('/root[@Somedate2 eq sql:variable("@DtAsString")]') 
     ,@x.exist('/root[(@Somedate2) eq xs:date(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[xs:date(@Somedate2) eq xs:date(sql:variable("@DtAsString"))]') 

     ,@x.exist('/root[xs:dateTime(@Somedate2) eq xs:dateTime(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[@Somedate2 cast as xs:dateTime? eq xs:dateTime(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[xs:date(@Somedate2) eq xs:date(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[@Somedate2 cast as xs:date? eq xs:date(sql:variable("@DtAsString"))]') 

     ,@x.exist('/root[@Somedate2 cast as xs:date? eq sql:variable("@DtAsString") cast as xs:date?]') 
     ,@x.exist('/root[@Somedate2 cast as xs:dateTime? eq sql:variable("@DtAsDateTime") cast as xs:dateTime?]') 

UNION ALL SELECT 
     @x.query('data(/root/@Somedate3)[1]') 
     ,@x.query('xs:date((/root/@Somedate3)[1])') 
     ,@x.query('xs:dateTime((/root/@Somedate3)[1])') 
     ,@x.query('(/root/@Somedate3)[1] cast as xs:date?') 
     ,@x.query('(/root/@Somedate3)[1] cast as xs:dateTime?') 
     ,@x.value('(/root/@Somedate3)[1]','date') 
     ,@x.value('(/root/@Somedate3)[1]','datetime') 
     ,@x.exist('/root[@Somedate3 eq sql:variable("@DtAsString")]') 
     ,@x.exist('/root[(@Somedate3) eq xs:date(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[xs:date(@Somedate3) eq xs:date(sql:variable("@DtAsString"))]') 

     ,@x.exist('/root[xs:dateTime(@Somedate3) eq xs:dateTime(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[@Somedate3 cast as xs:dateTime? eq xs:dateTime(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[xs:date(@Somedate3) eq xs:date(sql:variable("@DtAsString"))]') 
     ,@x.exist('/root[@Somedate3 cast as xs:date? eq xs:date(sql:variable("@DtAsString"))]') 

     ,@x.exist('/root[@Somedate3 cast as xs:date? eq sql:variable("@DtAsString") cast as xs:date?]') 
     ,@x.exist('/root[@Somedate3 cast as xs:dateTime? eq sql:variable("@DtAsDateTime") cast as xs:dateTime?]') 

結果

+---------------------+-------------+---------------------+-------------+---------------------+------------+-------------------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+ 
| SimpleData   | DateCtor | DtTmCtor   | CastToDate | ToDateTime   | Val2Date | Val2DtTime    | Equals1 | Equals2 | Equals3 | Equals4 | Equals5 | Equals6 | Equals7 | Equals8 | Equals9 | 
+---------------------+-------------+---------------------+-------------+---------------------+------------+-------------------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+ 
| 2002-01-01Z   | 2002-01-01Z |      | 2002-01-01Z |      | 2002-01-01 | 2002-01-01 00:00:00.000 | 1  | 1  | 1  | 0  | 0  | 1  | 1  | 1  | 0  | 
+---------------------+-------------+---------------------+-------------+---------------------+------------+-------------------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+ 
| 2002-01-01   | 2002-01-01 |      | 2002-01-01 |      | 2002-01-01 | 2002-01-01 00:00:00.000 | 0  | 1  | 1  | 0  | 0  | 1  | 1  | 1  | 0  | 
+---------------------+-------------+---------------------+-------------+---------------------+------------+-------------------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+ 
| 2002-01-01T00:00:00 |    | 2002-01-01T00:00:00 |    | 2002-01-01T00:00:00 | 2002-01-01 | 2002-01-01 00:00:00.000 | 0  | 0  | 0  | 0  | 0  | 0  | 0  | 0  | 1  | 
+---------------------+-------------+---------------------+-------------+---------------------+------------+-------------------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+