Compare XML to string? - sql-server

Question

I have a table with an XML column. The vast majority of rows contain this data:
<X C="1"></X>
I would like to find any row among the millions that does not match this. So...
select *
from DataResults
where cast(baserentamount as varchar(250)) not like '<X C="1"></X>'
This returns every row. I assume there is something very simple wrong here?
Update: perhaps I should reverse the sense of this question - what is the easiest way to do the query I want, IE, match the entry in an XML column in a WHERE?

Answer 1

You should look at the actual XML in your table. If you insert that xml into a table it is not the same string you think it is.
declare #Something table(baserentamount xml)
insert #Something select '<X C="1"></X>'
select *
, cast(baserentamount as varchar(250))
from #Something
Therefore your query will very likely return every row. And it will certainly return the rows you want to exclude because the xml string has changed.

Answer 2

Ahhh, well there it is, the devil is in the details:
XML as presented:
<X C="1" />
XML after CAST:
<X C="1"/>
Not entirely obvious, nor obvious why this should be, but there it is, it's a WS problem.

Answer 3

Any approach to do this on string level is wrong (due to different XML layouts, which are semantically equal and therefore possible at any stage).
Try something along this:
DECLARE #tbl TABLE (ID INT IDENTITY PRIMARY KEY, xmldata XML);
INSERT INTO #tbl (xmldata) VALUES
(N'<X C="1"></X>')
,(N'<X A="huh?" C="1"></X>')
,(N'<X C="2"></X>')
,(N'<X C="1"></X><X C="2"></X>')
,(N'<Y C="1"></Y>')
,(N'<root>some other</root>')
;
SELECT t.*
,CASE WHEN t.xmldata.query(N'count(/*)>1').value('.','bit')=1 THEN 'X' END MoreThanOneNodeInLevel1
,CASE WHEN t.xmldata.query(N'count(/*[1]/#*)>1').value('.','bit')=1 THEN 'X' END MoreThanOnAttributeInFirstElement
,CASE WHEN t.xmldata.query(N'/X[1]/#C != "1"').value('.','bit')=1 THEN 'X' END Attribute_X_C_isNot1
,CASE WHEN t.xmldata.query(N'local-name(/*[1])!="X"').value('.','bit')=1 THEN 'X' END FirstElementIsNotNamedX
FROM #tbl t;
The result
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| ID | xmldata | MoreThanOneNodeInLevel1 | MoreThanOnAttributeInFirstElement | Attribute_X_C_isNot1 | FirstElementIsNotNamedX |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| 1 | <X C="1" /> | NULL | NULL | NULL | NULL |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| 2 | <X A="huh?" C="1" /> | NULL | X | NULL | NULL |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| 3 | <X C="2" /> | NULL | NULL | X | NULL |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| 4 | <X C="1" /><X C="2" /> | X | NULL | NULL | NULL |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| 5 | <Y C="1" /> | NULL | NULL | NULL | X |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
| 6 | <root>some other</root> | NULL | NULL | NULL | X |
+----+-------------------------+-------------------------+-----------------------------------+----------------------+-------------------------+
As you can see, just the first row is NULL in all columns.
The idea in short:
Your XML has got
- 1 single element at level 1
- This element has got 1 single attribute
- The "C"-attribute's value in the first element="1"
- The local-name of the first element is "X"
In the sample data I added various "wrong" XML instances.
You might need to add more rules.
Hint: You can place the expression in a WHERE clause or use XML.exist().

Answer 4

The pure XML way to handle what you need would be to create a tiny XML Schema. After that you can validate that XML column against it.
SQL
USE tempdb;
GO
-- DDL and sample data population, start
IF EXISTS (SELECT * FROM sys.xml_schema_collections
WHERE name = N'MySchema'
AND schema_id = SCHEMA_ID(N'dbo'))
DROP XML SCHEMA COLLECTION dbo.MySchema;
CREATE XML SCHEMA COLLECTION dbo.MySchema
AS N'<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element name="X">
<xsd:complexType>
<xsd:attribute name="C" type="xsd:int" use="required" fixed="1"/>
</xsd:complexType>
</xsd:element>
</xsd:schema>'
GO
DECLARE #validationTbl TABLE (ID INT NOT NULL, Reason NVARCHAR(1024) NULL);
DECLARE #tbl TABLE (ID INT IDENTITY PRIMARY KEY, xmldata XML);
INSERT INTO #tbl (xmldata) VALUES
(N'<X C="1"></X>') -- good
,(N'<X C="1"/>') -- good
,(N'<X C="2"></X>') -- bad
,(N'<root>Miami</root>'); -- ugly
-- DDL and sample data population, end
-- Method #1
DECLARE #ID INT, #XML AS XML(dbo.MySchema)
, #RowCount INT = (SELECT COUNT(*) FROM #tbl);
WHILE #RowCount > 0
BEGIN
BEGIN TRY
SELECT #ID = ID, #XML = XMLData
FROM #tbl
ORDER BY ID DESC
OFFSET #RowCount - 1 ROWS FETCH NEXT 1 ROWS ONLY;
END TRY
BEGIN CATCH
INSERT INTO #validationTbl (ID, Reason)
VALUES (#ID, ERROR_MESSAGE());
END CATCH
SET #RowCount -= 1;
END;
-- test
SELECT * FROM #validationTbl;
-- Method #2
-- unfortunately, it stops at the very first error
-- TRY_CAST() shall swallow all XSD validation errors internally
-- and produce a NULL value for all invalid rows
/*
Validate Expressions (XQuery)
https://learn.microsoft.com/en-us/sql/xquery/validate-expressions-xquery?view=sql-server-ver15
https://learn.microsoft.com/en-us/sql/t-sql/functions/try-cast-transact-sql?view=sql-server-ver15
*/
SELECT TOP(4) *, TRY_CAST(xmldata AS XML(dbo.MySchema)) AS Result
FROM #tbl
ORDER BY ID;
Output
+----+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ID | Reason |
+----+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 2 | XML Validation: Element or attribute 'C' was defined as fixed, the element value has to be equal to value of 'fixed' attribute specified in definition. Location: /*:X[1]/#*:C |
| 3 | XML Validation: Declaration not found for element 'root'. Location: /*:root[1] |
+----+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+