Why this Join-Update query update all value? - sql

Question

I made some mistake and all values of a column are updated.
I did this in SQL Server 2008 R2.
I should have run some query like this:
UPDATE TABLE_A
SET FEEL = 'HAPPY'
FROM TABLE_A A
INNER JOIN TABLE_B B ON A.SN = B.SN
WHERE B.WEATHER = 'SUNNY';
However, I made a mistake and ran this:
UPDATE TABLE_A
SET FEEL = 'HAPPY'
FROM TABLE_C A
INNER JOIN TABLE_B B ON A.SN = B.SN
WHERE B.WEATHER = 'SUNNY';
and even TABLE_C has column of [SN].
I expected that this query update FEEL of TABLE_A as 'HAPPY' where WEATHER of TABLE_B is 'SUNNY' with inner join between two tables, but every value of column FEEL is updated to 'HAPPY'.
What means Update A set ~ from c in SQL Server and when it should be used? And why "inner-join" updates all values?

Answer 1

This query:
UPDATE TABLE_A
SET FEEL = 'HAPPY'
FROM TABLE_C A INNER JOIN
TABLE_B B
ON A.SN = B.SN
WHERE B.WEATHER = 'SUNNY';
is saying to update all rows in TABLE_A that match the conditions in the ON and WHERE clauses. But, none of those conditions involve TABLE_A. So, nothing is being filtered. Actually, what you are doing is equivalent to:
UPDATE AA
SET FEEL = 'HAPPY'
FROM TABLE AA CROSS JOIN
TABLE_C A INNER JOIN
TABLE_B B
ON A.SN = B.SN
WHERE B.WEATHER = 'SUNNY';
This is a bit of weirdness in the UPDATE.
When you do:
UPDATE TABLE_A
SET FEEL = 'HAPPY'
FROM TABLE_A A INNER JOIN
TABLE_B B
ON A.SN = B.SN
WHERE B.WEATHER = 'SUNNY';
SQL Server makes an exception to the rule that an alias always replaces the table reference. It still allows TABLE_A in the UPDATE to refer to A. So, there is no CROSS JOIN.
Personally, I consider this broken-ness, because a table alias should always replace the table reference. The developers at Microsoft think otherwise. And there is no standard that guides this syntax.
If you have a FROM clause, I recommend that always use table aliases.
UPDATE A
SET FEEL = 'HAPPY'
FROM TABLE_A A INNER JOIN
TABLE_B B
ON A.SN = B.SN
WHERE B.WEATHER = 'SUNNY';