Develop Reference

Best approach to track Amount field on Invoice table when InvoiceItem items change?

I'm building an app where I need to store invoices from customers so we can track who has paid and who has not, and if not, see how much they owe in total. Right now my schema looks something like this:
Customer
- Id
- Name
Invoice
- Id
- CreatedOn
- PaidOn
- CustomerId
InvoiceItem
- Id
- Amount
- InvoiceId
Normally I'd fetch all the data using Entity Framework and calculate everything in my C# service, (or even do the calculation on SQL Server) something like so:
var amountOwed = Invoice.Where(i => i.CustomerId == customer.Id)
.SelectMany(i => i.InvoiceItems)
.Select(ii => ii.Amount)
.Sum()
But calculating everything every time I need to generate a report doesn't feel like the right approach this time, because down the line I'll have to generate reports that should calculate what all the customers owe (sometimes go even higher on the hierarchy).
For this scenario I was thinking of adding an Amount field on my Invoice table and possibly an AmountOwed on my Customer table which will be updated or populated via the InvoiceService whenever I insert/update/delete an InvoiceItem. This should be safe enough and make the report querying much faster.
But I've also been searching some on this subject and another recommended approach is using triggers on my database. I like this method best because even if I were to directly modify a value using SQL and not the app services, the other tables would automatically update.
My question is:
How do I add a trigger to update all the parent tables whenever an InvoiceItem is changed?
And from your experience, is this the best (safer, less error-prone) solution to this problem, or am I missing something?

There are many examples of triggers that you can find on the web. Many are poorly written unfortunately. And for future reference, post DDL for your tables, not some abbreviated list. No one should need to ask about the constraints and relationships you have (or should have) defined.
To start, how would you write a query to calculate the total amount at the invoice level? Presumably you know the tsql to do that. So write it, test it, verify it. Then add your amount column to the invoice table. Now how would you write an update statement to set that new amount column to the sum of the associated item rows? Again - write it, test it, verify it. At this point you have all the code you need to implement your trigger.
Since this process involves changes to the item table, you will need to write triggers to handle all three types of dml statements - insert, update, and delete. Write a trigger for each to simplify your learning and debugging. Triggers have access to special tables - go learn about them. And go learn about the false assumption that a trigger works with a single row - it doesn't. Triggers must be written to work correctly if 0 (yes, zero), 1, or many rows are affected.
In an insert statement, the inserted table will hold all the rows inserted by the statement that caused the trigger to execute. So you merely sum the values (using the appropriate grouping logic) and update the appropriate rows in the invoice table. Having written the update statement mentioned in the previous paragraphs, this should be a relatively simple change to that query. But since you can insert a new row for an old invoice, you must remember to add the summed amount to the value already stored in the invoice table. This should be enough direction for you to start.
And to answer your second question - the safest and easiest way is to calculate the value every time. I fear you are trying to solve a problem that you do not have and that you may never have. Generally speaking, no one cares about invoices that are of "significant" age. You might care about unpaid invoices for a period of time, but eventually you write these things off (especially if the amounts are not significant). Another relatively easy approach is to create an indexed view to calculate and materialize the total amount. But remember - nothing is free. An indexed view must be maintained and it will add extra processing for DML statements affecting the item table. Indexed views do have limitations - which are documented.
And one last comment. I would strongly hesitate to maintain a total amount at any level higher than invoice. Above that level one frequently wants to filter the results in any ways - date, location, type, customer, etc. At this level you are approaching data warehouse functionality which is not appropriate for a OLTP system.

First of all never use triggers for business logic. Triggers are tricky and easily forgettable. It will be hard to maintain such application.
For most cases you can easily populate your reporting data via entity framework or SQL query. But if it requires lots of joins then you need to consider using staging tables. Because reporting requires data denormalization. To populate staging tables you can use SQL jobs or other schedule mechanism (Azure Scheduler maybe). This way you won't need to work with lots of join and your reports will populate faster.

Approach to storing and retrieving time when having multiple systems talking to each other

Consider this scenario with two applications whose data is synced
Application 1 | Application 2
Data from application1 is inserted/updated into application2 based on last modified date of records.
How can we make sure that the last modified date is not dependent on time zones.

I would use a rowversion column in both applications. To decide what to sync you can have a query or a merge statement and in the where clause you can compare the rows table1.RowVersion > table2.RowVersion. (table1 is from application1 and table2 is from application2)

This might help: ToUniversalTime
Convert the times to universal times, these take into account the time zone, daylight savings, etc. If you convert both times before adding the records, you should be able to get the correct last modified date.

How to have a database trigger code

I need some help with this..
This table I have has a date column in it, and when any of the dates in that column equal the servers date I need to tell my website/program to send out an email or perform some certain notification action to let the user know something.
I was thinking of having a program running on the server polling the data base a certain intervals but the problem with this is if the date is 01/31/11 10:30 AM and my interval is every 5 mins there potential for the polling to be inaccurate i.e. the poll polling at 10:35 AM. In other words I need the database to somehow notify something when "x" date has been hit exactly at that date.
I'd like to avoid having a 1sec interval checking the database as I think that would be a huge performance hit.
I'm using ASP.NET MVC 3 with MSSQL and LINQ Entity framework.
Any creative ideas?

You could use Quartz.net to setup those events. Quartz is pretty flexible and powerful - and it was meant for this sort of thing.

Do not have the database trigger the code. Have a trigger create a row in another table with information about what just happened.
Have a separate program periodically read from the second table to email users or whatever you need to do. Have that program delete the row from the table once it's done with the email.

I don't have any personal experience, but Sql Server CLR Integration might be the answer you are looking for. From the description it sounds like you can write almost anything that will compile against the .NET framework and deploy it to a sql server instance and Sql Server will be able to execute it. http://msdn.microsoft.com/en-us/library/ms254498.aspx

you either need to make use of a scheduler (e.g. DBMS_SCHEDULER in Oracle or SQL Server Jobs, etc) or find some third party tool like maybe Quartz.net as mentioned by another responder. Or maybe code something like the following into a polling app
select all jobs due in next 5 minutes, order by due date
while there are jobs
if the next job is due action it
else sleep for duration of interval till job due
loop

This is bit dirty, but I think it will give you the functionality you're looking for.
In Global.asax.cs
public DateTime LastMaxDateTime;
protected void Application_Start(object sender,EventArgs e)
{
LastMaxDateTime = GetMaxDateTime();
Thread bgThread=new Thread(BackgroundThread_CheckDatabase);
bgThread.IsBackground=true;
bgThread.Start();
}
private void BackgroundThread_CheckDatabase()
{
while(true)
{
DateTime dtMaxDateTime = GetMaxDateTime();
if(dtMaxDateTime > this.LastMaxDateTime)
{
//Send Notifications
this.LastMaxDateTime=dtMaxDateTime;
}
Thread.Sleep(5000); //5 seconds
}
}
private DateTime GetMaxDateTime()
{
//function that returns DateTime from something like "SELECT MAX(DateTimeColumn) FROM [MyTable]"
}
Basically, the code keeps track of the newest DateTime in your table and on each poll, checks to see if there's a newer DateTime in the database since the last time it checked. If so, you can send out your notifications. If you're not expecting many records in your table that could cause a race condition, then I don't see a problem with this as a quick solution.

Most efficient way to do it is to have an application that instead of polling runs event-driven.
For example, have a thread query the database for the earliest scheduled event and sleep until then. Then have another thread synchronously wait for a table change (e.g. in PostgreSQL this would be the NOTIFY/LISTEN statements) and signal the first thread to check if the earliest event has changed.

The easiest way is to keep track of the date of your last check. When you check again, pull all rows greater than the last check date and less than or equal to the new check date. To make sure you execute them, you could add a column for when the action was performed and update that. With an index on that new column there shouldn't be any performance problem with checking it every second for rows with a NULL DateExecuted.
You could also read ahead and sort the upcoming items by trigger date and do a Thread.Wait() until the next one comes up to be precise.

What is the best approach to calculate a formula which changes value each day?

I use the following columns stored in a SQL table called tb_player:
Date of Birth (Date), Times Played (Integer), Versions (Integer)
to calculate a "playvalue" (integer) in the following formula:
playvalue = (Today - Date of Birth) * Times Played * Versions
I display upto 100 of these records with the associataed playvalue on a webpage at any time.
My question is, what is the most efficient way of calculating this playvalue given it will change only once a day, due to the (today-date of birth) changing? The other values (times played & versions) remain the same.
Is there a better way than calculating this on the fly each time for the 100 records? If so, is it more efficient to do the calculation in a stored proc or in VB.NET/C#?

In a property/method on the object, in C#/VB.NET (your .NET code).
The time to execute a simple property like this is nothing compared to the time to call out-of-process to a database (to fetch the rows in the first place), or the transport time of a web-page; you'll never notice it if just using it for UI display. Plus it is on your easily-scaled-out hardware (the app server), and doesn't involve a huge update daily, and is only executed for rows that are actually displayed, and only if you actually query this property/method.

Are you finding that this is actually causing a performance problem? I don't imagine it would be very bad, since the calculation is pretty straightforward math.
However, if you are actually concerned about it, my approach would be to basically set up a "playvalue cache" column in the tb_player table. This column will store the calculated "playvalue" for each player for the current day. Set up a cronjob or scheduled task to run at midnight every day and update this column with the new day's value.
Then everything else can simply select this column instead of doing the calculation, and you only have to do the calculation once a day.

SQL Column name for indicating a change

I didn't really know how to formulate the title.
But this is my problem.
We are using SqlCacheDependencies to update out cache objects. But the query behind this is "complicated" that we would like to make it somewhat easier.
Our first thought was to add a fieldname like DateChanged and use the DateChanged to check if an object has actually changed in stead of loading all the columns of a table.
SELECT DateChanged FROM Table1
in stead of
SELECT Id, Title, DateChanged, Description FROM Table1
But I was hoping someone could tell me if there are other ways to do this or if there are standards for naming the column that indicates a change.
How are frameworks like "Entity framework" or "NHibernate" handle this?

It's usually called " timestamp"
ie in
LLBLGEN
LINQTOSQL

These are the columns I use on "auditable" entities:
version: hibernate's optimistic concurrency control
createdBy: FK to users
modifiedBy: FK to users
createdAt: timestamp
modifiedAt: timestamp

If you're on SQL Server be aware a timestamp column is somewhat misleadingly named. Whilst it can be used to track changes to a column, its value has nothing to do with the date or time. It can be thought of as an ID uniqiue within the database, nothing more. http://msdn.microsoft.com/en-us/library/aa260631.aspx
In order to know if a change has taken place you need two timstamps, like in cherouvim's answer. One is set when an object is created, the other on any modification, probably via update trigger. If they differ then the object has been edited, but you can't tell exactly when.
On SQL Server you have to use the same pattern, but with datetime columns. You can then calculate the date of the row by subtracting the creation date from the mod date.