SQL UPDATE from multiple rows to single row - sql

Question

this illustrates the issue:
CREATE TABLE Table_A (id int, value char)
INSERT INTO Table_A VALUES (1, 'A')
INSERT INTO Table_A VALUES (2, 'B')
CREATE TABLE Table_B (id int, value char)
INSERT INTO Table_B VALUES (1, 'C')
INSERT INTO Table_B VALUES (1, 'D')
If you run
UPDATE a SET a.value = (SELECT b.value FROM Table_B b WHERE a.id = b.id)
FROM Table_A a, Table_B b WHERE a.id = b.id
You get an error saying
Subquery returned more than 1 value. This is not permitted when the
subquery follows =, !=, <, <= , >, >= or when the subquery is used as
an expression.
But if you run this
UPDATE a SET a.value = b.value
FROM Table_A a, Table_B b WHERE a.id = b.id
No error is thrown and you get the row updated, why is that?
Edit:
Sorry guys, you seem to focusing on explaining why the first query gives error, but I think that is obvious and to me that is a desire result (because setting value of Table_A for id 1 to value of Table_B with id 1 is undefined when there are multiple values in Table_B with id 1)
My question is actually asking why the second query does not give you an error, which is causing trouble to me (i.e. I want it to break if I have more than one row with the same id but different values)

Answer 1

You got that error because you are using subquery when you set a new value and the subquery return more than 1 result.
SET a.value = (SELECT b.value FROM Table_B b WHERE a.id = b.id)
It will error when update a value with id = 1, because there is two record that have id = 1 in table b.
So your query will look like this (this is only for illustration and of course will cause an error)
UPDATE a SET a.value = ('C', 'D')
FROM Table_A a, Table_B b WHERE a.id = b.id
When you're using this query
UPDATE a SET a.value = b.value
FROM Table_A a, Table_B b WHERE a.id = b.id
You are join the table a with table b using id field, so the result is
a.id => a.value => b.value : 1 A C
a.id => a.value => b.value : 1 A D
No entry record for id = 2 because there is no matching record in table b.
So your update query will looks like this
UPDATE a SET a.value = 'C'
FROM Table_A a, Table_B b WHERE a.id = 1
UPDATE a SET a.value = 'D'
FROM Table_A a, Table_B b WHERE a.id = 1

Answer 2

Because your subquery will return both more than 1 result. The Assign statement will not accept more than 1 value.
You have to use JOIN
May be something like this
UPDATE A
SET A.value = B.value
FROM Table_A A INNER JOIN Table_B B ON A.id = B.id
FIDDLE DEMO

Answer 3

The reason your second query does not error our is because it will assign the first available B.value to a.value where it satisfies the condition
A.id = B.id
Try the following snippet and you will see how the Update...Set works.
create table #temp (id int, name varchar(100), phone int)
create table #temp1 (id int, phone int)
insert into #temp (id, name)
select 1, 'Mark' union
select 2, 'JOhn' union
select 3, 'Jerry'
insert into #temp1 (id, phone)
select 1, 123456 union
select 1, 222564 union
select 1, 222210
select * from #temp
select * from #temp1
update t
set phone = t1.phone
from #temp1 t1, #temp t
where t.id = t1.id
select * from #temp