如何从XML *向SQL Server DATE字段*中插入NULL

Question

I've got some XML that I'm trying to insert into a Microsoft SQL Server database using their XML datatype functions .

One of the table fields is a nullable DATE column. If the node is missing, then it's inserted as NULL which is great. However, if the node is present but empty <LastDay/> when running the XPath query, it interprets the value from the empty node as an empty string '' instead of NULL . So when looking at the table results, it casts the date to 1900-01-01 by default.

I would like for empty nodes to also be inserted as NULL instead of the default empty string '' or 1900-01-01. How can I get it to insert NULL instead?

CREATE TABLE myxml 
(
    "id" INT,
    "name" NVARCHAR(100),
    "company" NVARCHAR(100),
    "lastday" DATE
);

DECLARE @xml XML =
'<?xml version="1.0" encoding="UTF-8"?>
<Data xmlns="http://example.com" xmlns:dmd="http://example.com/data-metadata">
    <Company dmd:name="Adventure Works Ltd.">
        <Employee id="1">
            <Name>John Doe</Name>
            <LastDay>2016-08-01</LastDay>
        </Employee>
        <Employee id="2">
            <Name>Jane Doe</Name>
        </Employee>
    </Company>
    <Company dmd:name="StackUnderflow">
        <Employee id="3">
            <Name>Jeff Puckett</Name>
            <LastDay/>
        </Employee>
        <Employee id="4">
            <Name>Ill Gates</Name>
        </Employee>
    </Company>
</Data>';    

WITH XMLNAMESPACES (DEFAULT 'http://example.com', 'http://example.com/data-metadata' as dmd)
INSERT INTO myxml (id,name,company,lastday)
SELECT 
    t.c.value('@id',         'INT' ),
    t.c.value('Name[1]',     'VARCHAR(100)' ),
    t.c.value('../@dmd:name','VARCHAR(100)' ),
    t.c.value('LastDay[1]',  'DATE' )
FROM @xml.nodes('/Data/Company/Employee') t(c)

This produces:

 id  name         company              lastday
 ------------------------------------------------
 1   John Doe     Adventure Works Ltd. 2016-08-01
 2   Jane Doe     Adventure Works Ltd. NULL
 3   Jeff Puckett StackUnderflow       1900-01-01
 4   Ill Gates    StackUnderflow       NULL

I am trying to achieve:

 id  name         company              lastday
 ------------------------------------------------
 1   John Doe     Adventure Works Ltd. 2016-08-01
 2   Jane Doe     Adventure Works Ltd. NULL
 3   Jeff Puckett StackUnderflow       NULL
 4   Ill Gates    StackUnderflow       NULL

Answer 1

You have to use NULLIF function to avoid default values popping out from XML selection.

Returns a null value if the two specified expressions are equal.

Your query will be changed as below:

SELECT 
    t.c.value('@id',         'INT' ),
    t.c.value('Name[1]','VARCHAR(100)' ),
    t.c.value('../@dmd:name',    'VARCHAR(100)' ),
    NULLIF(t.c.value('LastDay[1]',  'DATE' ),'')
FROM @xml.nodes('/Data/Company/Employee') t(c)

For more information on NULLIF , please check this MSDN page .

Answer 2

Besides techspider's very good answer I'd like to show another approach:

Doing .nodes() on Company and CROSS APPLY .nodes() on Employee allows a cleaner XPath navigation and avoids the backward navigation you are using by ../@dmd.name . In your case this is just for info probably, but good to consider: If there was a company without any Employee you would skip the whole company otherwise... (My code would skip as well due to the CROSS APPLY , but you could use OUTER APPLY ).

And to your actual question: Using the internal cast as xs:date will do the logic within the XQuery and should be faster...

WITH XMLNAMESPACES (DEFAULT 'http://example.com', 'http://example.com/data-metadata' as dmd)
INSERT INTO myxml (id,name,company,lastday)
SELECT 
    e.value('@id',          'INT' ),
    e.value('Name[1]',      'VARCHAR(100)' ),
    c.value('@dmd:name',    'VARCHAR(100)' ),
    e.value('let $x:=LastDay[1] return $x cast as xs:date?','DATE' )
FROM @xml.nodes('/Data/Company') AS A(c) 
CROSS APPLY c.nodes('Employee') AS B(e)