Getting results based on condition from another table in SQL SERVER

Question

I have got two tables for a Microhydel Application, one consists the monthly billing record of Consumers from which monthly customer Bill is generated.

CREATE TABLE billing_history(
    [id] [numeric](18, 0) IDENTITY(1,1) NOT NULL,
    [reading_date] [date] NOT NULL,
    [reading_year] [smallint] NULL,
    [reading] [numeric](18, 0) NOT NULL,
    [consumer_id] [int] NOT NULL,
    [paid_amount] [numeric](18, 0) NULL)

I have another table which stores the different slab for per unit cost for both commercial and domestic users.

CREATE TABLE [rate_list](
    [flag] [varchar](50) NULL,
    [limit] [numeric](18, 0) NOT NULL,
    [price] [numeric](18, 0) NOT NULL,
    [service_charges] [numeric](18, 0) NOT NULL,
    [surcharge] [numeric](18, 0) NOT NULL
) 

For eg For a Domestic customer consuming 50 units or less electricity monthly will be charged differently then a Commercial customer consuming the same amount of electricity. Similarly consuming units over this slab will have another rate applied on them.

Thanks to @bluefeet i already have the query to generate the numbers of units consumed from the first table using the query

select c.consumer_id, 
  sum(c.reading - isnull(pre.reading, 0)) TotalReading
from
(
  select consumer_id,
    reading,
    month(getdate()) curMonth,
    year(getdate()) curYear,
    case when month(getdate()) = 1 then 12 else month(getdate()) -1 end preMonth,
    case when month(getdate()) = 1 then year(getdate())-1 else year(getdate()) end preYear
  from billing_history
  where month(reading_date) = month(getdate())
    and year(reading_date) = year(getdate())
) c
left join billing_history pre
  on c.consumer_id = pre.consumer_id
  and month(pre.reading_date) = c.preMonth
  and year(pre.reading_date) = c.preYear
group by c.consumer_id;

However I need to generate Monthly bill for each customer so that for eg according to the rates in the rate_list table. The Key here is DOMESTIC/COMMERCIAL which has different slabs for the number of units consumed.

Any ideas

Answer 1

A few comments on my answer.

First, I wasn't not sure where the type_of_connection flag is present in the SQL Fiddle that you posted so I added it to Consumers .

Second, I think you need to alter your price_list2 table to include the limit start and end values for the prices. Otherwise it will be very difficult to determine the price for each consumer.

I used the following price_list2 table which will contain the start/end values for each limit:

CREATE TABLE [price_list2](
    [flag] [varchar](50) NULL,
    [limitStart] [numeric](18, 0) NOT NULL,
    [limitEnd] [numeric](18, 0) NOT NULL,
    [price] [numeric](18, 0) NOT NULL,
    [service_charges] [numeric](18, 0) NOT NULL,
    [surcharge] [numeric](18, 0) NOT NULL);

On to the query, using the tables and the original query that you posted you should be able to use something like this:

select c.consumer_id,
  r.totalreading * p.price TotalBill
from Consumers c
inner join
(
  select c.consumer_id, 
    sum(c.reading - isnull(pre.reading, 0)) TotalReading
  from
  (
    select consumer_id,
      reading,
      month(getdate()) curMonth,
      year(getdate()) curYear,
      case when month(getdate()) = 1 then 12 else month(getdate()) -1 end preMonth,
      case when month(getdate()) = 1 then year(getdate())-1 else year(getdate()) end preYear
    from billing_history
    where month(reading_date) = month(getdate())
      and year(reading_date) = year(getdate())
  ) c
  left join billing_history pre
    on c.consumer_id = pre.consumer_id
    and month(pre.reading_date) = c.preMonth
    and year(pre.reading_date) = c.preYear
  group by c.consumer_id
) r
  on c.consumer_id = r.consumer_id
inner join price_list2 p
  on c.type_of_connection = p.flag
  and r.totalreading between p.limitStart and p.limitEnd

See SQL Fiddle with Demo

As you can see when joining on the price_list2 table I am joining on the start/end range of the limits. This allows you to determine what price should be used for the bill.