UPDATE with a one-to-many JOIN (multiple columns) - sql-server

Question

I've run into an issue and I'm not sure if it is the intended behaviour. I have searched online for an answer, but all I could find was about what row would update last and so what would be the value after the query. I know there is no such thing as order in that case, and you can't be sure what the value would be ahead of time.
In my case though, I'm updating different columns, so overriding a previous update is not a concern of mine.
CREATE TABLE #original (id int, value1 int, value2 int)
INSERT INTO #original (id) VALUES (1), (2)
CREATE TABLE #temp (id int, name varchar(10), value int)
INSERT INTO #temp (id, name, value) VALUES (1, 'value1', 10), (1, 'value2', 11), (2, 'value1', 20), (2, 'value2', 21)
SELECT * FROM #original
id value1 value2
----------- ----------- -----------
1 NULL NULL
2 NULL NULL
SELECT * FROM #temp
id name value
----------- ---------- -----------
1 value1 10
1 value2 11
2 value1 20
2 value2 21
UPDATE O SET
value1 = CASE WHEN T.name = 'value1' THEN T.value ELSE value1 END,
value2 = CASE WHEN T.name = 'value2' THEN T.value ELSE value2 END
FROM
#original O
INNER JOIN #temp T ON T.id = O.id
SELECT * FROM #original
id value1 value2
----------- ----------- -----------
1 10 NULL
2 20 NULL
I don't get why both value2 are NULL.
SELECT
O.id,
CASE WHEN T.name = 'value1' THEN T.value ELSE value1 END AS value1,
CASE WHEN T.name = 'value2' THEN T.value ELSE value2 END AS value2
FROM
#original O
INNER JOIN #temp T ON T.id = O.id
id value1 value2
----------- ----------- -----------
1 10 NULL
1 NULL 11
2 20 NULL
2 NULL 21
Running the above instead of the update, it looks exactly like what I thought it would and I would assume it means "four" updates, populating both value1 and value2 in both rows.
I would really appreciate if someone could explain this one to me.

Answer 1

The problem here is your assumption: "In my case though, I'm updating different columns, so overriding a previous update is not a concern of mine." It most certainly is a problem, because ultimately, your code
value1 = CASE WHEN T.name = 'value1' THEN T.value ELSE value1 END,
value2 = CASE WHEN T.name = 'value2' THEN T.value ELSE value2 END
only runs (or takes effect) once per row. SQL Server is not required to update the same row twice, and will normally just arbitrarily take a single row's values, one of which is NULL.
The documentation states (my bold):
Use caution when specifying the FROM clause to provide the criteria for the update operation. The results of an UPDATE statement are undefined if the statement includes a FROM clause that is not specified in such a way that only one value is available for each column occurrence that is updated, that is if the UPDATE statement is not deterministic. For example, in the UPDATE statement in the following script, both rows in Table1 meet the qualifications of the FROM clause in the UPDATE statement; but it is undefined which row from Table1 is used to update the row in Table2.
Only one row may be used and one update done, you cannot assume that the updates will happen sequentially. So you need to make sure you have a single match for each row you want to update.
Therefore you should pre-aggregate your values
UPDATE O SET
value1 = T.value1,
value2 = T.value2
FROM
#original O
INNER JOIN (
SELECT
id,
MAX(CASE WHEN T.name = 'value1' THEN T.value END) value1,
MAX(CASE WHEN T.name = 'value2' THEN T.value END) value2,
FROM #temp T
GROUP BY
id
) T ON T.id = O.id;
You could also use CROSS APPLY or a CTE for this.

Answer 2

Modified the update clause with a little tweak in getting the value1 and value2
;with cte as (SELECT
O.id,
max(CASE WHEN T.name = 'value1' THEN T.value ELSE value1 END) AS value1,
max(CASE WHEN T.name = 'value2' THEN T.value ELSE value2 END) AS value2
FROM
#original O
INNER JOIN #temp T ON T.id = O.id
group by o.id
)
UPDATE o SET
o.value1 = i.value1 ,
o.value2 = i.value2
FROM
#original o
INNER JOIN cte i ON i.id = O.id