XQuery adding or replacing attribute in single SQL update command

Question

I have a Table with an XML column, I want to update the xml to insert attribute or to change the attribute value if the attribute already exists.

Let's say the starting xml is: < d />

Inserting:

UPDATE Table 
set XmlCol.modify('insert attribute att {"1"} into /d[1]')

Changing:

UPDATE Table
set XmlCol.modify('replace value of /d[1]/@att with "1"')

insert will fail if the attribute already exists, replace will fail if the attribute doesn't exists. I have tried to use 'if' but I don't think it can work, there error I get: "XQuery [modify()]: Syntax error near 'attribute', expected 'else'."

IF attempt

UPDATE Table 
set XmlCol.modify('if empty(/d[1]/@att) 
                   then insert attribute att {"1"} into /d[1]
                   else replace value of /d[1]/@att with "1"')

Currently I select the xml into a variable and then modify it using T-SQL and then updating the column with new xml, this requires me to lock the row in a transaction and is probably more expensive for the DB.

Answer 1

From what I can tell, you can't do this with single statement. You can use the exist() method to accomplish that with two update statements.

DECLARE @TestTable TABLE
(
    Id int,
    XmlCol xml
);

INSERT INTO @TestTable (Id, XmlCol)
VALUES
    (1, '<d att="1" />'),
    (2, '<d />'),
    (3, '<d att="3" />');

SELECT * FROM @TestTable;

UPDATE @TestTable
SET XmlCol.modify('replace value of /d[1]/@att with "1"')
WHERE XmlCol.exist('(/d[1])[not(empty(@att))]') = 1;

UPDATE @TestTable
SET XmlCol.modify('insert attribute att {"1"} into /d[1]')
WHERE XmlCol.exist('(/d[1])[empty(@att)]') = 1;

SELECT * FROM @TestTable;

The output from the final select is:

Id          XmlCol
----------- -------------------
1           <d att="1" />
2           <d att="1" />
3           <d att="1" />