Oracle: Update with join not working as expected - sql

Question

Edit: The answer given by mathguy works perfectly. But I really wanted to understand why the first update statement isn't working and second one does. I know that + operator is not suggested, but in this case as the second table was in subquery, I had to use it.
Short Question. (Detailed explanation and Create/Insert statements below)
What is the difference between these 2 update statement and why first one is not working as expected, whereas second one does.
update d_dim d
set FLAG=
(select case when t.id is null then 'N' else 'Y' end as FLAG
from t_temp t
where d.id=t.id(+)
);
And
update d_dim d
set FLAG=
(select case when t.id is null then 'N' else 'Y' end as FLAG
from t_temp t,d_dim d1
where d1.id=t.id(+)
and d1.id=d.id
);
Detailed Explanation
I am trying to replicate a workplace scenario. Unfortunately SQLFiddle for Oracle is not working so couldn't create fiddle demo.
I have 2 tables, d_dim(ID,FLAG) and t_temp(ID) like below
select * from d_dim;
+----+------+
| ID | FLAG |
+----+------+
| 1 | |
| 2 | |
| 3 | |
| 4 | |
+----+------+
select * from t_temp;
+----+
| ID |
+----+
| 1 |
| 3 |
+----+
Now I need to set the FLAG in d_dim as Y or N.
If ID exists in t_temp the set it Y. Else set it N.
So expected output should be like.
+----+------+
| ID | FLAG |
+----+------+
| 1 | Y |
| 2 | N |
| 3 | Y |
| 4 | N |
+----+------+
This is the update statement I am using (Using (+) as in this case I need left join from d_dim to t_temp
update d_dim d
set FLAG=
(select case when t.id is null then 'N' else 'Y' end as FLAG
from t_temp t
where d.id=t.id(+)
)
--4 rows updated.
But ID 2 and 4 are updated as NULL.
select * from d_dim;
+----+------+
| ID | FLAG |
+----+------+
| 1 | Y |
| 2 | |
| 3 | Y |
| 4 | |
+----+------+
If I use just the select clause after plugging in d_dim table, I get correct output.
select d.id,
case when t.id is null then 'N' else 'Y' end as FLAG
from t_temp t,d_dim d
where d.id=t.id(+)
order by id
+----+------+
| ID | FLAG |
+----+------+
| 1 | Y |
| 2 | N |
| 3 | Y |
| 4 | N |
+----+------+
I did some hit and trial and came up with this query, which seems to be working
update d_dim d
set FLAG=(select case when t.id is null then 'N' else 'Y' end as FLAG
from t_temp t,d_dim d1
where d1.id=t.id(+)
and d1.id=d.id);
select * from d_dim;
+----+------+
| ID | FLAG |
+----+------+
| 1 | Y |
| 2 | N |
| 3 | Y |
| 4 | N |
+----+------+
So my question is that
Why the initial update statement doesn't work properly and why is it
updating null for id 2 and 4.
Please find the Create and Insert statements below
CREATE TABLE d_dim (id int, flag varchar2(4));
INSERT ALL INTO d_dim (id, flag) VALUES (1, NULL)
INTO d_dim (id, flag) VALUES (2, NULL)
INTO d_dim (id, flag) VALUES (3, NULL)
INTO d_dim (id, flag) VALUES (4, NULL)
SELECT * FROM dual;
CREATE TABLE t_temp (id int)
;
INSERT ALL
INTO t_temp (id) VALUES (1)
INTO t_temp (id) VALUES (3)
SELECT * FROM dual;

Answer 1

In the second query you have an outer join. In the first query you don't have any kind of join; you simply have a select from t, with a where clause where there is a (+) after t.id. I don't know why that syntax doesn't return an error; but when d.id doesn't exist in t, that subquery returns no rows, and that's how update works when the update value is supposed to be the output of a scalar subquery: if the subquery returns no rows, the update statement will update the field with NULL.
You didn't ask for a different way to make the update work, but if you want to see one, here it is. No doubt you know how to do this; offering it for the benefit of other forum members.
update d_dim
set FLAG = case when id in (select id from t_temp) then 'Y' else 'N' end;
EDIT: It seems the OP didn't fully understand my point so here are more details.
The Oracle documentation states explicitly:
•The (+) operator does not produce an outer join if you specify one table in the outer query and the other table in an inner query.
https://docs.oracle.com/cd/B28359_01/server.111/b28286/queries006.htm
(under the heading "Outer Joins", after the "See Also" box)

Answer 2

An update statement would update based on whether the condition was satisfied for the update as specified in the subselect clause.
When you mention
update d_dim d
set FLAG=
(select case when t.id is null then 'N' else 'Y' end as FLAG
from t_temp t
where d.id=t.id(+)
);
This statement would be executed only when d.id is equal to t.id. In your case, for id=2, d.id is 2, t.id is null. 2 is NOT equal to null, so the update has no effect on id 2.