XQuery adding or replacing attribute in single SQL update command - sql-server

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" />

Answer 2

There is a slightly better way than Tommys:
DECLARE #TestTable TABLE
(
Id int,
XmlCol xml
);
INSERT INTO #TestTable (Id, XmlCol)
VALUES
(1, '<UserSettings> </UserSettings>'),
(2, '<UserSettings><timeout>3</timeout> </UserSettings>'),
(3, '<UserSettings> </UserSettings>');
UPDATE #TestTable
SET XmlCol.modify('replace value of (/UserSettings/timeout/text())[1] with "1"')
WHERE Id = 3 and XmlCol.exist('/UserSettings/timeout') = 1;
IF ##ROWCOUNT=0
UPDATE #TestTable
SET XmlCol.modify('insert <timeout>5</timeout> into (/UserSettings)[1] ')
WHERE Id = 3;
SELECT * FROM #TestTable;
The solution is a combination of Tommys and simple SQL and required only 1 SQL UPDATE if the column exist. Tommys always recuire two updates.