This is relatively trivial to do with a correlated subquery. You can't use the COALESCE method highlighted in the blog post you mention unless you extract that to a user-defined function (or unless you only want to return one row at a time). Here is how I typically do this:
DECLARE @x TABLE
(
id INT,
row_num INT,
customer_code VARCHAR(32),
comments VARCHAR(32)
);
INSERT @x SELECT 1,1,'Dilbert','Hard'
UNION ALL SELECT 1,2,'Dilbert','Worker'
UNION ALL SELECT 2,1,'Wally','Lazy';
SELECT id, customer_code, comments = STUFF((SELECT ' ' + comments
FROM @x AS x2 WHERE id = x.id
ORDER BY row_num
FOR XML PATH('')), 1, 1, '')
FROM @x AS x
GROUP BY id, customer_code
ORDER BY id;
If you have a case where the data in comments could contain unsafe-for-XML characters (>, <, &), you should change this:
FOR XML PATH('')), 1, 1, '')
To this more elaborate approach:
FOR XML PATH(''), TYPE).value(N'(./text())[1]', N'varchar(max)'), 1, 1, '')
(Be sure to use the right destination data type, varchar or nvarchar, and the right length, and prefix all string literals with N if using nvarchar.)
My only suggestion is to create stored procedure that performs DML on tag_bundle_relation and restrict all users from manipulating the table directly (it's in a sense 'explicit lock' as you say). I don't see any other acceptable ways to enforce the required restriction. In my understanding your simple solution just adds complexity to the system - you will need more triggers to synchronize tags field with tag_bundle_relation and tag tables. Also, you may get false fail if you are adding records to tag_bundle_relation one by one - for instance, inserting tag_bundle_id=3 with tag_ids 100,101,102 will fail after attempting to insert 101 (assuming 100 already inserted).
Best Answer