SQL - Optimize nested queries in Redshift - sql

Question

hope to pick on the wisdom of the folks here. (current approach at the bottom)
Please see the SQL Fiddle Link below.
http://sqlfiddle.com/#!17/4a818/1
create table paymentmethod (
methodtype varchar(6) not null,
methodname varchar(6) not null
);
insert into paymentmethod values
('cc', 'mstr'),
('cc', 'visa'),
('cc', 'amex')
;
create table chargepolicy (
accountname varchar(6) not null,
methodtype varchar(6) not null,
methodname varchar(6) not null,
chargerate float not null
);
insert into chargepolicy values
('acct1','cc', 'mstr', 0.02),
('acct1','cc', 'visa', 0.02),
('acct1','cc', 'amex', 0.025),
('acct2','cc', 'all', 0.015),
('acct3','cc', 'amex', 0.03),
('acct3','cc', 'all', 0.02),
('acct4','cc', 'visa', 0.0),
('acct4','cc', 'all', 0.025)
;
select * from chargepolicy, paymentmethod
where chargepolicy.methodtype = paymentmethod.methodtype
and (chargepolicy.methodname = paymentmethod.methodname
or chargepolicy.methodname = 'all')
order by 1,2,3;
So, this a hypothetical case (obviously). I have payment methods like Visa,MasterCard, Amex and then charge rates for each of these methods. The charge rates will vary base on the account. These accounts to method & rate mapping is defined in another table.
Now then, when I have charge rates defined for each method, things work fine (e.g. acct1 and acct2). Things get insteresting - and this is where I need help - when we come to acct3 and acct4. In acct3, I am saying that for Amex the charge should be 3% and for everything else (all - Amex) it should be 2%. Similarly, in acct4, I want charge for Visa to be 0% and for everything else to be 2.5%.
The way current SQL is written, I am getting two values for acct3.amex and acct4.visa.
How can I simplify that. It must be a straight forward thing but I don't believe I am looking at it right.
This is what I have come up with:
http://sqlfiddle.com/#!17/36a1c/6
select *
from
chargepolicy,
paymentmethod
where
chargepolicy.methodtype = paymentmethod.methodtype
and (chargepolicy.methodname = paymentmethod.methodname
or (chargepolicy.methodname = 'all'
and paymentmethod.methodname not in (
select methodname from chargepolicy cp2 where cp2.accountname = chargepolicy.accountname
and cp2.methodname !='all')
))
order by 1,2,3;
But since my actual use case has a lot of additional options to be mapped (e.g. Country, Product, Currency etc), I am not sure if my current approach is scalable - too many nested queries.

Answer 1

My approach would be like this, its much simpler to follow i think.
select
ac.accountname,
pm.methodtype,
pm.methodname,
coalesce(cp1.chargerate,cp2.chargerate) as chargerate
from
paymentmethod as pm
join (select distinct accountname from chargepolicy) as ac on 1=1
left join chargepolicy as cp1 on pm.methodname=cp1.methodname and ac.accountname=cp1.accountname
left join chargepolicy as cp2 on cp2.methodname='all' and ac.accountname=cp2.accountname
order by 1,2,3;
http://sqlfiddle.com/#!17/36a1c/11