Re: How to update a table with the result of deleting rows in another table

> On 6 Oct 2020, at 7:37, Hemil Ruparel <hemilruparel2002@gmail.com> wrote:
>
> I am trying to delete orders for a given customer on a given date and add the cost of those orders to credit for the
customer. 
>
> So far, I came up with this:
> ```
> with data as (
>     delete from orders
>         where customer_id = <customer id>
>     and date = '2020-10-05' returning price
> ), total as (
>     select sum(price) from data
> )
> update paymentdetail
> set temp_credit = temp_credit + (select * from total)
> where customer_id = <customer id>
> ```
>
> which works. but is there a better way to update one table using the result of deleting rows from another table given
thatI only want the aggregate of the result? 

Adding the customer id to your returning clause and using update..from could help:

with data as (
    delete from orders
    where customer_id = <customer id>
    returning customer_id, price
), total as (
    select customer_id, sum(price) as total_price
    from data
    group by customer_id
)
update paymentdetail
set temp_credit = temp_credit + total.total_price
from total
where customer_id = total.customer_id


You could also do this using subqueries instead of CTE’s, that may perform better as CTE’s act as optimisation fences.


Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.