SQL Update with join - sql

Question

Is there any other faster way to do an update besides with a join? Here's my query but it's a bit slow:
UPDATE #user_dupes
SET ExternalEmail = ud2.Email
FROM #user_dupes ud1
INNER JOIN(
SELECT Email, UserName
FROM #user_flat_emailtable_dupes
WHERE EmailType = 2
AND Email IS NOT NULL AND LEN(Email) > 0
) ud2
ON ud1.UserName = ud2.UserName
Thanks for any ideas

Answer 1

If you are using SQL Server, you were almost there. It's just a little fix:
UPDATE ud1 --little fix here!
SET ExternalEmail = ud2.Email
FROM #user_dupes ud1
INNER JOIN
(
SELECT Email, UserName
FROM #user_flat_emailtable_dupes
WHERE EmailType = 2
AND Email IS NOT NULL AND LEN(Email) > 0
) ud2
ON ud1.UserName = ud2.UserName

Answer 2

A couple of changes, on top of what #Adrian said...
UPDATE
ud1 -- #Adrian's change. Update the instance that you have already aliased.
SET
externalEmail = ud2.Email
FROM
#user_dupes AS ud1
INNER JOIN
#user_flat_emailtable_dupes AS ud2
ON ud1.UserName = ud2.UserName
WHERE
ud2.EmailType = 2 -- Removed sub-query, for layout, doubt it will help performance
AND ud2.Email IS NOT NULL
AND ud2.Email <> '' -- Removed the `LEN()` function
But possibly the most important past is to ensure you have indexes. The JOIN is necessary for this logic (or correlated sub-queries, etc), so you want the join to be performant.
An Index of (UserName) on #user_dupes, and an Index of (EmailType, Email, UserName) on #user_flat_emailtable_dupes. (This assumes ud2 is the smaller table, after the filtering)
With the indexes as specified, the change from LEN(Email) > 0 to Email <> '' may allow an index seek rather than scan. The larger your tables the more apparent this will be.

Answer 3

I believe this query will do the same thing (although you'd have to be sure to form #user_flat_emailtable_dupes with no duplicate usernames). I haven't checked to see if they have different execution plans. It looks like you're refining junky input, I mention this partly because I do a lot of that and find MERGE useful (all the more useful for me since I don't know how UPDATE FROM works). And partly because I hadn't ever used MERGE with variables. It appears to be the case that at least the target table must be aliased, or the parser decides #ud1 is a scalar variable and it breaks.
MERGE #user_dupes AS ud1
USING #user_flat_emailtable_dupes AS ud2
ON emailType = 2
AND COALESCE(ud2.email, '') <> ''
AND ud2.username = ud1.username
WHEN MATCHED THEN UPDATE
SET externalEmail = ud2.email
;