T-SQL String Replace is Not Locking - Last Update Wins - sql

Question

I have the following stored procedure, which is intended to iterate through a list of strings, which contains several substrings of the form prefix.bucketName. I want to iterate through each string and each bucket name, and replace the old prefix with a new prefix, but keep the same bucket name.
To give an example, consider this original string:
"(OldPrefix.BucketA)(OldPrefix.BucketB)"
So for example I would like to get:
"(NewPrefix.BucketA)(NewPrefix.BucketB)"
What I actually get is this:
"(OldPrefix.BucketA)(NewPrefix.BucketB)"
So, in general, only one of the prefixes get updated, and it is not predictable which one. Based on some investigation I have done, it appears that both replacements actually work, but only the last one is actually saved. It seems like SQL should be locking this column but instead, both are read at the same time, the replace is applied, and then both are written, leaving the last write as what shows in the column.
Here is the query - All variable names have been changed for privacy - Some error handling and data validation code was left out for brevity:
DECLARE #PrefixID INT = 1478,
DECLARE #PrefixName_OLD NVARCHAR(50) = 'OldPrefix',
DECLARE #PrefixName_NEW NVARCHAR(50) = 'NewPrefix'
BEGIN TRAN
-- Code to rename the section itself here not shown for brevity
UPDATE
dbo.Component
SET
AString= REPLACE(AString,'('+#Prefix_OLD+'.'+b.BucketName+')', '('+#PrefixName_NEW+'.'+b.BucketName+')'),
FROM
dbo.Component sc
JOIN
dbo.ComponentBucketFilterInString fis
ON
sc.ComponentID = fis.ComponentID
JOIN
dbo.Buckets b
ON
fis.BucketID = b.BucketID
WHERE
b.PrefixID = #PrefixID
COMMIT
RETURN 1
When I write the same query using a while loop, it performs as expected:
DECLARE #BucketsToUpdate TABLE
(
BucketID INT,
BucketName VARCHAR(256)
)
INSERT INTO #BucketsToUpdate
SELECT BucketID, BucketName
FROM Buckets WHERE PrefixID = #PrefixID
WHILE EXISTS(SELECT 1 FROM #BucketsToUpdate)
BEGIN
DECLARE #currentBucketID INT,
#currentBucketName VARCHAR(256)
SELECT TOP 1 #currentBucketID = bucketID, #currentBucketName = bucketName FROM #BucketsToUpdate
UPDATE
dbo.Component
SET
AString = REPLACE(AString,'('+#PrefixName_OLD+'.'+#currentBucketName+')', '('+#PrefixName_NEW+'.'+#currentBucketName+')')
FROM
dbo.Component sc
JOIN
dbo.ComponentBucketFilterInString fis
ON
sc.ComponentID = fis.ComponentID
WHERE fis.BucketID = #currentBucketID
DELETE FROM #BucketsToUpdate WHERE BucketID = #currentBucketID
END
Why does the first version fail? How can I fix it?

Answer 1

The problem you are experiencing is "undefined" behavior when there is more than single match possible for UPDATE FROM JOIN.
In order to make your update possible you should run it multiple times updating one pair of values at a time as you proposed in your second code demo.
Related: How is this script updating table when using LEFT JOINs? and Let’s deprecate UPDATE FROM!:
SQL Server will happily update the same row over and over again if it matches more than one row in the joined table, >>with only the result of the last of those updates sticking<<.

Answer 2

Not sure why you are making the whole process so complex. May be I am not clearly understanding the requirement. As per my understanding, you are looking to update only Prefix part for column 'AString' in the table dbo.Component. Current value for example is-
(OldPrefix.BucketA)(OldPrefix.BucketB)
You wants to update the value as-
(NewPrefix.BucketA)(NewPrefix.BucketB)
Am I right? If yes, you can update all records with a simple Update script as below-
DECLARE #PrefixID INT = 1478
DECLARE #PrefixName_OLD NVARCHAR(50) = 'OldPrefix'
DECLARE #PrefixName_NEW NVARCHAR(50) = 'NewPrefix'
UPDATE Component
SET AString= REPLACE(AString,#PrefixName_OLD,#PrefixName_NEW)