使用不同的表 SQL 添加计算列

Question

I'm new to SQL so please bare with.

I am trying to add a calculated column called 'InvoiceAmount' to my 'CompanyInvoice' table.

My CompanyInvoice table currently has the following columns:

CREATE TABLE CompanyInvoice(
InvoiceID        CHAR(5) NOT NULL,
CompanyAccountID CHAR(5) NOT NULL CONSTRAINT FK_CompanyInvoice_CompanyAccount    
                                  FOREIGN KEY REFERENCES CompanyAccount(CompanyAccountID) ON UPDATE CASCADE,
CompanyID        CHAR(5) NOT NULL CONSTRAINT FK_CompanyInvoice_ProductionCompany 
                                  FOREIGN KEY REFERENCES ProductionCompany(CompanyID), 
BookingID        CHAR(5) NOT NULL CONSTRAINT FK_CompanyInvoice_EventBooking      
                                  FOREIGN KEY REFERENCES EventBooking (BookingID),
TheatreID        CHAR(5) NOT NULL CONSTRAINT FK_CompanyInvoice_Theatre           
                                  FOREIGN KEY REFERENCES Theatre(TheatreID)ON UPDATE CASCADE);

I would like to add a new column which calculates the invoice amount - invoice amount is calculated dependant on duration of event (EventDuration column is held in a different 'Event' table) and the 'TheatreID' for example if TheatreID=1 then EventDuration x 300, if TheatreID=2 then EventDuration x 1000.

Many Thanks

Answer 1

A view, as you described, would look like

create view v_invoice as
select c.InvoiceID,
       e.EventDuration * case when c.TheatreID = 1 then 300
                              when c.TheatreID = 2 then 1000
                         end as invoice_amount
from CompanyInvoice c join ... ??? unknown; how to get to Event table?

As you can see, ??? means that you didn't describe how to get from data we know to Event table.

---

On the other hand, it is probably not a very good idea to multiply anything with hardcoded values ( 300 , 1000 ) because - what if they change? Will you modify all code you write to implement those changes? From what you said so far, those should be another column in Theatre table (or a new table, if you want to track changes over time; that new table would be in parent-child relationship with Theatre ).

Something like this (note that I'm still guessing join conditions):

create view v_invoice as
select c.InvoiceID,
       e.EventDuration * t.value as invoice_amount
                         -------
                         this, instead of hardcoded 300 or 1000 or ...
from CompanyInvoice c join Theatre t on t.TheatreID = c.TheatreID
                      join Event e on e.EventID = t.EventID