Bill Calculation using SQL - sql

Question

I've having some problems as I'm trying to calculate the total amount for a consumption.
so for example:
for the first 20 watt is charged at 0.40
for the next 20 watt is charged at 0.80
and 0.90 for each watt in excess of 20 watt
I've tried doing something on my own but the calculation is wrong and is there any way to optimize the query as i do not wish to do the calculation at the main select statement. is subquery a better option?
I'm using Oracle as DB engine
SELECT CustomerName, previousRead, newRead, newRead-previousRead AS Consumption,
(((newRead-previousRead-100)*FR)+((newRead-previousRead-200)*SR)+((newRead-previousRead-200)/200)*TR)as TotalBill
FROM (
SELECT C.firstName||''||C.lastName as CustomerName, R.newReading previousRead,
lead(R.newReading) OVER (PARTITION BY R.meterID ORDER BY R.dateVisited) AS newRead, B.firstconsumptionRate as FR,
B.secondconsumptionRate as SR, B.firstconsumptionRate as TR
FROM reading R, serviceaddress S, electricmeter W, customer C, rate A, nonresidentialrate B
WHERE W.meterID = S.meterID
AND R.meterID = W.meterID
AND A.rateID = S.rateID
AND B.rateID = A.rateID
AND C.custID = S.custID
AND C.custType = 'Non-residential'
AND r.datevisited >= TO_DATE('01-06-2014','DD-MM-YYYY')
AND r.datevisited < TO_DATE('31-07-2014','DD-MM-YYYY')
)where newRead is not null;
Table
CREATE TABLE Customer(
custID INTEGER NOT NULL,
firstName CHAR(25) NOT NULL,
lastName CHAR(25) NOT NULL,
NRIC CHAR(9) NOT NULL,
custType CHAR(25) NOT NULL,
badStatus CHAR(25) Default 'Good',
CONSTRAINT cust_Pkey PRIMARY KEY (custID),
CONSTRAINT cust_type CHECK (custType IN ('Residential', 'Non-residential')),
CONSTRAINT custBadStatus_type CHECK (badStatus IN ('Late Payment', 'Non Payment', 'Good'))
);
CREATE TABLE Reading (
readingID INTEGER NOT NULL,
meterID INTEGER NOT NULL,
dateVisited DATE NOT NULL,
newReading NUMBER(10,0) NOT NULL,
CONSTRAINT reading_Pkey PRIMARY KEY (readingID),
CONSTRAINT reading_AltKey UNIQUE (meterID, dateVisited),
CONSTRAINT reading_meterID_Fkey FOREIGN KEY (meterID) REFERENCES Meter (meterID),
CONSTRAINT checkReading CHECK (newReading > 0)
);
CREATE TABLE Rate (
rateID INTEGER NOT NULL,
rateApprDate DATE NOT NULL,
rateEffDate DATE NOT NULL,
rateType CHAR(25) NOT NULL,
CONSTRAINT rate_Pkey PRIMARY KEY (rateID),
CONSTRAINT rateType CHECK (rateType IN ('Residential', 'Non-residential')),
CONSTRAINT validDate CHECK (rateApprDate < rateEffDate),
);
CREATE TABLE NonResidentialRate (
rateID INTEGER NOT NULL,
firstconsumptionRate NUMBER(5,2) NOT NULL,
secondconsumptionRate NUMBER(5,2) NOT NULL,
thirdconsumptionRate NUMBER(5,2) NOT NULL,
CONSTRAINT nonResidentialRate_Pkey PRIMARY KEY (rateID),
CONSTRAINT nonResidentialRate_rateID_FK FOREIGN KEY (rateID) REFERENCES Rate (rateID)
);
CREATE TABLE ServiceAddress (
svcAddID INTEGER NOT NULL,
meterID INTEGER NOT NULL,
custID INTEGER NOT NULL,
rateID INTEGER NOT NULL,
street CHAR(30) NOT NULL,
city CHAR(35) NULL,
state CHAR(2) NULL,
zipPostalCode CHAR(9) NOT NULL,
CONSTRAINT serviceAddress_Pkey PRIMARY KEY (svcAddID),
CONSTRAINT serviceAddress_meterID_Fkey FOREIGN KEY (meterID) REFERENCES Meter (meterID),
CONSTRAINT serviceAddress_custID_Fkey FOREIGN KEY (custID) REFERENCES Customer (custID),
CONSTRAINT serviceAddress_rateID_Fkey FOREIGN KEY (rateID) REFERENCES Rate (rateID),
);
CREATE TABLE Meter (
meterID INTEGER NOT NULL,
SerialNum CHAR(30) NOT NULL,
installationDate DATE NOT NULL,
CONSTRAINT waterMeter_Pkey PRIMARY KEY (meterID),
CONSTRAINT waterMeter_Altkey UNIQUE (SerialNum)
);
The customer can be both of residential and non-residential and each of them has a service address. The meter is unique to each service address and the reading is stored in a different table.
There are 2 kinds of rates as well, their are different kind of rates for residential and nonresidential.
As for the reading, I did not create a new attribute for previous reading because it can be obtained tru the previous month reading
hope this makes my question clearer.

Answer 1

Here you go, you can give it a try..
SELECT CustomerName, previousRead, newRead, newRead-previousRead AS Consumption,
CASE WHEN newRead-previousRead <= 100
THEN (newRead-previousRead)*FR
WHEN newRead-previousRead <= 200
THEN 100*FR + (newRead-previousRead-100)*SR
WHEN newRead-previousRead > 200
THEN 100*FR + 100*SR + ((newRead-previousRead)-200)*TR
END as TotalBill
FROM (
SELECT C.firstName||''||C.lastName as CustomerName, R.newReading previousRead,
lead(R.newReading) OVER (PARTITION BY R.meterID ORDER BY R.dateVisited) AS newRead, B.firstconsumptionRate as FR,
B.secondconsumptionRate as SR, B.thirdconsumptionRate as TR
FROM reading R, serviceaddress S, meter W, customer C, rate A, nonresidentialrate B
WHERE W.meterID = S.meterID
AND R.meterID = W.meterID
AND A.rateID = S.rateID
AND B.rateID = A.rateID
AND C.custID = S.custID
AND C.custType = 'Non-residential'
AND r.datevisited >= TO_DATE('01-06-2014','DD-MM-YYYY')
AND r.datevisited < TO_DATE('31-07-2014','DD-MM-YYYY')
)where newRead is not null;

Answer 2

Were it me, I'd probably use a CASE statement and structure the code for clarity rather than trying to eke out every last cycle of performance (unless you're really in an environment where you need every last cycle)
(CASE WHEN newRead-previousRead <= 20
THEN (newRead-previousRead)*0.40
WHEN newRead-previousRead <= 40
THEN 20*0.40 + (newRead-previousRead-20)*0.80
WHEN newRead-previousRead > 40
THEN 20*0.40 + 20*0.80 + (newRead-previousRead-40)*0.90
END)
Now, obviously, there are ways to optimize this (by, for example, computing newRead-previousRead in the inner query once rather than potentially needing to compute it 4 times in the CASE statement). And this is not a particularly flexible option if you want to be able to change the billing structure frequently (in which case the tiers and rates would need to be in a separate table that you read). But it's hopefully a relatively clear expression of the computation.