SQL update statement with different table in from clause - sql

Question

Out of accident I noticed that the following query is actually valid:
UPDATE bikes
SET price = NULL
FROM inserted
WHERE inserted.owner_id = 123456
This is part of a trigger where someone forgot to join the original table to the inserted table. The result is that when the trigger is executed, all prices are set to NULL.
The correct SQL statement is this:
UPDATE bikes
SET price = NULL
FROM inserted
INNER JOIN bikes ON bikes.id=inserted.id
WHERE inserted.owner_id = 123456
How/why is this first statement valid?

Answer 1

Why wouldn't it be valid? SQL Server doesn't know what you're trying to do. It thinks you want to update all of the fields where some condition exists on another table. See the last update below.
SETUP
declare #table table
(
id int,
name varchar(10)
)
declare #itable table
(
id int,
name varchar(10)
)
insert into #table (id, name)
select 1,'abc' union
select 2,'def' union
select 3,'ghi' union
select 4,'jkl' union
select 5,'mno' union
select 6,'pqr'
insert into #itable (id, name)
select 1,'abc' union
select 2,'def' union
select 3,'ghi' union
select 4,'jkl' union
select 5,'mno' union
select 6,'pqr'
All names on #table will change to zzz
update #table
set name = 'zzz'
from #itable i
where i.id = 1
select * from #itable
select * from #table
All names where id = 1 on #table becomes yyy
update #table
set name = 'yyy'
from #itable i
inner join #table t on i.id = t.id
where i.id = 1
select * from #itable
select * from #table
This will NOT update anything
update #table
set name = 'aaa'
from #itable i
where i.id = 133
select * from #itable
select * from #table

Answer 2

The first statement does not work as expected because it is missing the entire INNER JOIN line with the bikes and inserted table. Without that SQL Server will update all rows as all rows will qualify for an update when the inserted.owner_id = 123456.
You can reproduce this outside of the trigger in TSQL like :
update bikes set price =null
from SomeOtherTable
where SomeOtherTable.SomeColumn = 'some_value_that_exists'
This is syntactically valid statement in SQL Server. If the Intention is to update bikes table based on existance of a row in some unrelated table that cant be joined because the 2 tables arent related then this is how you would do it. But that is not your requirement. Hence why it updates all records instead of only those that match the bikes.id In programming terms this is called as a logical bug.
The inner join makes it more restrictive and forces to to update only those rows that match the join condition between the 2 tables (bikes.id=inserted.id comparison )

Answer 3

In Simple terms the from clause is optional..Consider below query..
update table
set id=10
This has one table right after update clause ,sql just updates it..
now consider below query..
update table1
set id=40
from table2..
What do you think SQL does,it updates all the rows same as first query..
unless you refer to another table in from clause and join like below
update t1
set t1.id=40
from
table1 t1
join
table2 t2
on t1.id=t2.id
below is the from clause explanation in update syntax stripped down to show only to relevant parts..
If the object being updated is the same as the object in the FROM clause and there is only one reference to the object in the FROM clause, an object alias may or may not be specified. If the object being updated appears more than one time in the FROM clause, one, and only one, reference to the object must not specify a table alias. All other references to the object in the FROM clause must include an object alias
As long as above rules are valid (as in your case),SQL will happily update table found immediately after update clause