Updating columns in Oracle based on values in other tables - sql

Question

I am fairly new to creating and altering tables in SQL (Oracle) and have a question involving updating one table based on values in others.
Say I have table A:
ID Date Status
--- --- ---
1 1/1/2000 Active
2 5/10/2007 Inactive
2 2/15/2016 Active
3 10/1/2013 Inactive
4 1/11/2004 Inactive
5 4/5/2012 Inactive
5 6/12/2014 Active
and table B:
ID Date Status Number of Records in A
--- --- --- ---
1
2
3
4
5
What is the best way to update table B to get the most recent Date and Status of each item and count of records in A? I know I could join tables but I would like B to exist as its own table.

Answer 1

Oracle lets you assign multiple columns at once in an update statement. So, you can do:
update b
set (dt, status, ct) =
(select max(dt),
max(status) keep (dense_rank first order by dt desc),
count(*)
from a
where a.id = b.id
) ;
You can basically use the subquery -- with a group by -- if you want the results for all ids as a query:
select max(dt),
max(status) keep (dense_rank first order by dt desc),
count(*)
from a
group by id;
You can also use create table as or insert into to put the records directly into b, without having to match them up using update.

Answer 2

Something like this. If you already have a table B and you need to populate it with the values from this query, or if you need to create a new table B with these values, adapt as needed. NOTE: I used dt as a column name, since "date" is a reserved word in Oracle. (For the same reason I used "ct" for "count".)
with
table_A ( id, dt, status ) as (
select 1, to_date( '1/1/2000', 'mm/dd/yyyy'), 'Active' from dual union all
select 2, to_date('5/10/2007', 'mm/dd/yyyy'), 'Inactive' from dual union all
select 2, to_date('2/15/2016', 'mm/dd/yyyy'), 'Active' from dual union all
select 3, to_date('10/1/2013', 'mm/dd/yyyy'), 'Inactive' from dual union all
select 4, to_date('1/11/2004', 'mm/dd/yyyy'), 'Inactive' from dual union all
select 5, to_date(' 4/5/2012', 'mm/dd/yyyy'), 'Inactive' from dual union all
select 5, to_date('6/12/2014', 'mm/dd/yyyy'), 'Active' from dual
),
prep ( id, dt, status, rn, ct ) as (
select id, dt, status,
row_number() over (partition by id order by dt desc),
count(*) over (partition by id)
from table_A
)
select id, to_char(dt, 'mm/dd/yyyy') as dt, status, ct
from prep
where rn = 1
;
ID DT STATUS CT
---------- ---------- -------- ----------
1 01/01/2000 Active 1
2 02/15/2016 Active 2
3 10/01/2013 Inactive 1
4 01/11/2004 Inactive 1
5 06/12/2014 Active 2
Added: You mentioned you are pretty new at this... so: for example, if you need to create table_B with these results, and table_A already exists and is populated: FIRST, you will not need the "table_A" factored subquery in my solution; and SECOND, you will create table_B with something like
create table table_B as
with
prep ( .....) -- rest of the solution here, up to and including the ;