Can I update a particular attribute of a tuple with the same attribute of another tuple of same table? If possible what should be the algorithm? - algorithm

Question

Suppose I have a table with 10 records/tuples. Now I want to update an attribute of 6th record with the same attribute of 1st record, 2nd-7th, 3rd-8th, 4th-9th, 5th-10th in a go i.e. without using cursor/loop. Use of any number of temporary table is allowed. What is the strategy to do so?

Answer 1

PostgreSQL (and probably other RDBMSes) let you use self-joins in UPDATE statements just as you can in SELECT statements:
UPDATE tbl
SET attr = t2.attr
FROM tbl t2
WHERE tbl.id = t2.id + 5
AND tbl.id >= 6

Answer 2

This would be easy with an update-with-join but Oracle doesn't do that and the closest substitute can be very tricky to get to work. Here is the easiest way. It involves a subquery to get the new value and a correlated subquery in the where clause. It looks complicated but the set subquery should be self-explanatory.
The where subquery really only has one purpose: it connects the two tables, much as the on clause would do if we could do a join. Except that the field used from the main table (the one being updated) must be a key field. As it turns out, with the self "join" being performed below, they are both the same field, but it is required.
Add to the where clause other restraining criteria, as shown.
update Tuples t1
set t1.Attr =(
select t2.Attr
from Tuples t2
where t2.Attr = t1.Attr - 5 )
where exists(
select t2.KeyVal
from Tuples t2
where t1.KeyVal = t2.KeyVal)
and t1.Attr > 5;
SqlFiddle is pulling a hissy fit right now so here the data used:
create table Tuples(
KeyVal int not null primary key,
Attr int
);
insert into Tuples
select 1, 1 from dual union all
select 2, 2 from dual union all
select 3, 3 from dual union all
select 4, 4 from dual union all
select 5, 5 from dual union all
select 6, 6 from dual union all
select 7, 7 from dual union all
select 8, 8 from dual union all
select 9, 9 from dual union all
select 10, 10 from dual;
The table starts out looking like this:
KEYVAL ATTR
------ ----
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 10
with this result:
KEYVAL ATTR
------ ----
1 1
2 2
3 3
4 4
5 5
6 1
7 2
8 3
9 4
10 5