how to conditionally update table - database

Question

I have one address table (containing k_id-PK, address) and one add_hist log table(containing k_id, address,change date) i.e. it has all address per id and on which date address change.
I want to make an update query which will update address column in address table so,fetching latest address from add_hist table will do the job.I am almost done with my query. Its fetching correct result too. But I want if address table is already updated, then dont update it.Here goes my query.Please review and correct it to get the desired result.
update address a set k_add =
(select kad from (
select h.k_id kid, h.k_add kad, h.chg_dt from add_hist h,
(select k_id, max(chg_dt) ch from add_hist
group by k_id
) h1
where h1.k_id = h.k_id
and h1.ch=h.chg_dt
) h2
where h2.kid = a.k_id)
;

Answer 1

You could use a merge instead of an update:
merge into address a
using (
select k_id, max(k_add) keep (dense_rank last order by chg_dt) as k_add
from add_hist
group by k_id
) h
on (a.k_id = h.k_id)
when matched then
update set a.k_add = h.k_add
where (a.k_add is null and h.k_add is not null)
or (a.k_add is not null and h.k_add is null)
or a.k_add != h.k_add;
The query in the using clause finds the most recent address for each ID from the history table. When a matching ID exists on the main table that is updated - but only if the value is different, because of the where clause.
With some dummy data:
create table address (k_id number primary key, k_add varchar2(20));
create table add_hist (k_id number, k_add varchar2(20), chg_dt date);
insert into address (k_id, k_add) values (1, 'Address 1');
insert into address (k_id, k_add) values (2, 'Address 2');
insert into address (k_id, k_add) values (3, null);
insert into address (k_id, k_add) values (4, null);
insert into add_hist (k_id, k_add, chg_dt) values (1, 'Address 1', date '2017-01-01');
insert into add_hist (k_id, k_add, chg_dt) values (1, 'Address 2', date '2017-01-02');
insert into add_hist (k_id, k_add, chg_dt) values (1, 'Address 1', date '2017-01-03');
insert into add_hist (k_id, k_add, chg_dt) values (2, 'Address 1', date '2017-01-01');
insert into add_hist (k_id, k_add, chg_dt) values (2, 'Address 2', date '2017-01-02');
insert into add_hist (k_id, k_add, chg_dt) values (2, 'Address 3', date '2017-01-03');
insert into add_hist (k_id, k_add, chg_dt) values (3, 'Address 1', date '2017-01-01');
insert into add_hist (k_id, k_add, chg_dt) values (3, null, date '2017-01-02');
insert into add_hist (k_id, k_add, chg_dt) values (4, 'Address 1', date '2017-01-01');
commit;
running your update statement gets:
4 rows updated.
select * from address;
K_ID K_ADD
---------- --------------------
1 Address 1
2 Address 3
3
4 Address 1
After rolling back to the starting state, running the merge gets:
2 rows merged.
select * from address;
K_ID K_ADD
---------- --------------------
1 Address 1
2 Address 3
3
4 Address 1
Same final result, but 1 row merged rather than 2 rows updated.
(If you run the merge without the where clause, all four rows are still affected; without the null checks only row with ID 2 is updated).

Answer 2

You can achieve the desired result with an UPDATE statement. Specifically, you need to "update through a join." The syntax has to be precise though. Update with joins
Using the same setup as in Alex's answer, the following update statement will update one row.
EDIT: See Alex Poole's comments below this Answer. The solution proposed here will work only in Oracle 12.1 and above. The problem is not the "update through join" concept, but the source rowset being the result of an aggregation. It has to do with the way in which Oracle knows, at compile time, that the "join" column in the source rowset is unique (it has no duplicates). In older versions of Oracle, an explicit unique or primary key constraint or index was required. Of course, when we GROUP BY <col>, the <col> will be unique in the result set of an aggregation, but it will not have a unique constraint or index on it. It seems Oracle recognized this situation, and since 12.1 it allows update through join where the source table is the result of an aggregation, as shown in this example.
update
( select a.k_add as current_address, q.new_address
from (
select k_id,
min(k_add) keep (dense_rank last order by chg_dt) as new_address
from add_hist
group by k_id
) q
join
address a on a.k_id = q.k_id
)
set current_address = new_address
where current_address != new_address
or current_address is null and new_address is not null
or current_address is not null and new_address is null
;