Oracle PL/SQL update based on values calculated from multiple tables - oracle

Question

I need to write a PL/SQL that sums up the value from 2 tables and update the value to another table.
I have the following tables: ONLINE_SALES, STORE_SALES, TOTAL_SALES
Assume the tables are structured like this:
ONLINE_SALES: OS_ID, STORE_ID, SEQ, ITEM_NAME, PRICE, PURCHASED_DATE
STORE_SALES: SS_ID, STORE_ID, SEQ, ITEM_NAME, PRICE, PURCHASED_DATE
TOTAL_SALES: STORE_ID, YEAR, TOTAL_INCOME
I want to write a PL/SQL that runs monthly and sums up the income (values in PRICE field) made in the month from both ONLINE_SALES and STORE_SALES of each store (identified by STORE_ID) and add the value to record in TOTAL_SALES with relative year.
My idea is to first filter the record by PURCHASED_DATE from both table with a SELECT, loop through all selected rows and sum to a variable and at last update the result with an UPDATE. But I am stuck in the first step since I found that I cannot use SELECT and only SELECT INTO is available.
Any ideas on how such PL/SQL can be written?

Answer 1

This solution assumes your stored procedure takes a DATE which it uses to identify the month and the year which is being totalled. Perhaps your assignment is expecting a different form of input? No worries. The deriving the values of year and date range is separate from the main process, so it is easy to swap in some different logic.
The loop uses an explicit cursor with the FOR UPDATE syntax. This locks the TOTAL_SALES table, which means you are guaranteed to be able to update all the rows.
create or replace calc_total_sales
( p_month in date )
is
cursor c_tot_sales (p_year number) is
select * from total_sales
where year = p_year
for update of total_income;
c_tot_sales c_tot_sales%rowtype;
l_os_sales number;
l_ss_sales number;
l_year number;
1_first_day date;
1_last_day date;
begin
l_year := to_number( to_char( p_month, 'yyyy') );
l_first_day := trunc( p_month, 'mm');
l_last_day := last_day( p_month);
open c_tot_sales( l_year );
loop
fetch c_tot_sales into r_tot_sales;
exit when c_tot_sales%not found;
select sum(price)
into l_os_sales
from online_sales
where store_id = r_tot_sales.store_id
and purchased_date >= l_first_day
and purchased_date <= l_last_day;
select sum(price)
into l_ss_sales
from store_sales
where store_id = r_tot_sales.store_id
and purchased_date >= l_first_day
and purchased_date <= l_last_day;
update total_sales
set total_income = total_income + nvl(l_ss_sales,0) + nvl(l_os_sales,0)
where current of c_total_sales;
end loop;
close c_tot_sales;
commit;
end;
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