Update table in Postgresql by grouping rows - sql

Question

I want to update a table by grouping (or combining) some rows together based on a certain criteria. I basically have this table currently (I want to group by 'id_number' and 'date' and sum 'count'):
Table: foo
---------------------------------------
| id_number | date | count |
---------------------------------------
| 1 | 2001 | 1 |
| 1 | 2001 | 2 |
| 1 | 2002 | 1 |
| 2 | 2001 | 6 |
| 2 | 2003 | 12 |
| 2 | 2003 | 2 |
---------------------------------------
And I want to get this:
Table: foo
---------------------------------------
| id_number | date | count |
---------------------------------------
| 1 | 2001 | 3 |
| 1 | 2002 | 1 |
| 2 | 2001 | 6 |
| 2 | 2003 | 14 |
---------------------------------------
I know that I can easily create a new table with the pertinent info. But how can I modify an existing table like this without making a "temp" table? (Note: I have nothing against using a temporary table, I'm just interested in seeing if I can do it this way)

Answer 1

If you want to delete rows you can add a primary key (for distinguish rows) and use two sentences, an UPDATE for the sum and a DELETE for obtain less rows.
You can do something like this:
create table foo (
id integer primary key,
id_number integer,
date integer,
count integer
);
insert into foo values
(1, 1 , 2001 , 1 ),
(2, 1 , 2001 , 2 ),
(3, 1 , 2002 , 1 ),
(4, 2 , 2001 , 6 ),
(5, 2 , 2003 , 12 ),
(6, 2 , 2003 , 2 );
select * from foo;
update foo
set count = count_sum
from (
select id, id_number, date,
sum(count) over (partition by id_number, date) as count_sum
from foo
) foo_added
where foo.id_number = foo_added.id_number
and foo.date = foo_added.date;
delete from foo
using (
select id, id_number, date,
row_number() over (partition by id_number, date order by id) as inner_order
from foo
) foo_ranked
where foo.id = foo_ranked.id
and foo_ranked.inner_order <> 1;
select * from foo;
You can try it here: http://rextester.com/PIL12447
With only one UPDATE
(but with a trigger) you can set a NULL value in count and trigger a DELETE in that case.
create table foo (
id integer primary key,
id_number integer,
date integer,
count integer
);
create function delete_if_count_is_null() returns trigger
language plpgsql as
$BODY$
begin
if new.count is null then
delete from foo
where id = new.id;
end if;
return new;
end;
$BODY$;
create trigger delete_if_count_is_null
after update on foo
for each row
execute procedure delete_if_count_is_null();
insert into foo values
(1, 1 , 2001 , 1 ),
(2, 1 , 2001 , 2 ),
(3, 1 , 2002 , 1 ),
(4, 2 , 2001 , 6 ),
(5, 2 , 2003 , 12 ),
(6, 2 , 2003 , 2 );
select * from foo;
update foo
set count = case when inner_order = 1 then count_sum else null end
from (
select id, id_number, date,
sum(count) over (partition by id_number, date) as count_sum,
row_number() over (partition by id_number, date order by id) as inner_order
from foo
) foo_added
where foo.id_number = foo_added.id_number
and foo.date = foo_added.date
and foo.id = foo_added.id;
select * from foo;
You can try it in: http://rextester.com/MWPRG10961