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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.

How can I properly present and write a checkboxlist to a db? (Theory/Logic help)

Please note that the database design I have now is fully in sandbox mode. Nothing is finalized. Everything (again this is in sandbox mode) is in one single table. Also, I'm in now way looking for coding help. I'm looking for the right theoretical/logical approach to this puzzle so I can go in and play with the coding myself. I learn better that way. If I need coding help, I'll come back here for further assistance.
I'm in the process of creating the first of a few CheckBoxList for a manually created form submittal. I've been looking over multiple ways to not only create said CheckBoxList but to enter it into the database. However, the challenge I'm facing (mainly because I haven't encountered this scenario before and I'd like to learn this) is that I not only need to worry about entering the items correctly in the database but I will eventually need to produce a report or make a printable form from these entries.
Let's say I have an order form for a specific type of Barbeque grill and I will need to send this form out to distriution centers across the nation. The distribution centers will need to pull said barbecues if they are highlighted on the form.
Here's what the CheckBoxList for the distibution centers will look like:
All
Dallas
Miami
Los Angeles
Seattle
New York
Chicago
Phoenix
Montreal
If the specific city (or all the cities) are checked, then the distribution center will pull the barbecue grill for shipment.
The added part is that I want to:
be able to create a grid view from this database for reporting to note which distribution center got orders for barbecues and
be able to create reports to tell what distribution center sent out barbecue orders in a given month (among other reporting).
Here's what I'm playing around with right now.
In my aspx page I have a checkboxlist programmed with all the distribution centers entered as a listitem as well as an option for 'ALL' (of the distribution centers).
I also created a dedicated column in this table that holds all the information in the listitem and programmed a sqldataconnection to this table to play with the programmability of leveraging the database for this purpose.
When it comes to writing the selections to the database, I originally created a column for each destination city including the 'All' option. I was toying around with just putting the selections into one single column but with some of the information I've been reading today about Database Normalization, the former options seems to be a better one than the latter. Is this correct practice for situations such as this especially if I need to think about reporting? Do I put the CheckBoxList data in one cell in a specific column? Do I create seprate columns for each distribution center? Am I even on the right track here?

Depending on the amount of cities that you want to store, I've used bitwise operators to store small amounts of data. Effectively, it would store it in the table like this:
CityID Location
2 Dallas
4 Miami
8 New York
16 Chicago
32 Montreal
and keep going in base 2 for additional cities.
When your user selects multiple cities for the order, the value that gets inserted into the database for cities is a bitwise OR calculation. So if they select Dallas, New York, and Chicago, you would be doing the following:
2 OR 8 OR 16
Which would equal 26
Now, you can use bitwise AND on the resulting value. So if checking for Miami the following is the evaluation:
26 AND 4 = 0
which indicates that Miami was not selected. Any value that was selected in the evaluation, it would return its ID like this:
26 AND 8 = 8
Again, I've only used this for small subsets of data, and to make the data storage as compact as possible. Computationally, it may be a trifle more expensive that some other methods, but I'm not 100% certain.

Note: This might not be the best approaches but I have seen them used.
1) Having one column of comma-delimited string
This should work well if the options don't have IDs in the database (having a separate referenced table)
You will need to loop through the checkbox list, obtained the selected options and concatenate them with String.Join()
You will need to split the string upon receiving it from the db and use it to check the checkboxes if there text is found in the resulting array
Problem: You might need a split function in the DB that converts the comma-separated string into rows. There split function implementation on the web/stackoverflow
2) You can have a separate table for the locations e.g. xxxxtable_location where the FK to the main table is referenced. This will be a one-many table
ParentID, Location
1 Dallas
2 Miami
2 New York
2 Chicago
3 Miami

Design of queryable web based results

I'm trying to accomplish something like a facebook news feed wall, loading N number of results from the overall dataset, starting with the most recent, date descending. When you click “more”, it displays the next N underneath and so on until you finish the dataset.
I’m struggling to come up with the best design to accomplish this. Ive always been told that stateless web services are the only way to build a scalable enterprise application, which means that as I understand it, keeping the whole results object cached serverside on the first call to the page, and just taking N results from it with each subsequent web service call is a no no?
If that’s the case, then something like GetResults(int pageindex, int pagesize) would work.... and thats how I WAS going to do it but then I realised it would not work if someone added a new DB record in between calls. Eg you start with 23 wall feed items in the DB and want to display them 10 at a time.
First call, page 1, page size 10 will return results 14-23 (most recent first)
Someone then adds 2 new posts, so you have 25 now in the DB
Second call, page 2, page size 10 will return results 6-15, two of which were already returned in the first call.
So this offsetting approach doesn’t work because you can’t guarantee the underlying dataset will remain the same between calls.
Im confused, how do I accomplish this?
Edit: Sorry a little more info. To avoid the problem of huge data table lookups, I had considered the option of pre-populating a "transient" table with the last few days data for that user when you first load the screen, then just reading the results a page at a time from that transient table to make it faster reading, with a slightly slower load time. Then when you exhaust that data, you bring in the next period (say 2 weeks) into the transient table and continue reading.
The difficulty is that users will "Post" items which then automatically will be picked up by users who match their search criteria. Eg if your criteria state you want to meet people between 25 and 32 and within 50 miles of you, then when you load up your news feed, you want it to show posts from all users who match your criteria. Kindof like a dynamic friends list.
How I was going to achieve this was at time of login, a stored proc would run which would populate a transient table in the DB by selecting all users and filtering down based on age and location criteria which I have in static lookup tables (postcode distances etc), then it will save the list of Users who match your criteria to this transient table for use whenever you then need to filter posts or search users. If you update your preferences, it will also recalculate this but only when you update prefs or re-login. So any new users signing up won't appear until you next login, which is fine I think.
Then when it comes time to display your news feed, all it does is retrieves this list of User Ids from the DB who match your criteria, then brings back all NewsFeedPosts which were posted by those users. Hey presto, dynamic news feed!
But obviously this is a subset of the entire NewsFeedPost table which is generated on the fly, so it doesn't make sense to recalculate this every time a user clicks "more", so this was how I was thinking about implementing it.
Tables - NewsFeedCurrent, NewsFeedRecent, NewsFeedArchive
New posts are created in the current table. Every night a batch job runs that moves all data from current that is 2 days old, to the recent table, and any data in the recent table that is a week old to the archive table.
The thinking being that 90% of the time, the user will only be interested in the last 2 days of data. So keep table small for access time. Another 9% of the time the user may want the last weeks data. So keep that separate in a secondary table. Then only 1% of the time the user wants data more than a week old so keep that in a larger, slow archive table that will be slower, but gives you performance boost by keeping current and recent tables small.
So when you first hit the news feed page, what it was going to do is take the pre-generated user list for your account and pull out all NewsFeedCurrent items and put them in a transient table, say TempNewsFeed under your user ID. You can then work with this resultset just by pulling back everything for your user id, no filtering required for items you arent interested in as they are pre-filtered. this will add a second or so to the page load but will improve response time when fetching results. Then when that data is exhausted, it will then - again using the list of users matching your criteria - pull out all relevant data from the Recent table, adding it to the TempNewsFeed table, allowing you to continue fetching data up to a week old. When thats exhausted, it will finally go to the archive table and using the user id list, pull out all data matching this and put in the temp table, allowing you to continue navigating the remaining data. This will give a fairly significant delay as it populates the archive data but if you are going back a week, then you will have to accept 5-10 seconds wait while it populates the data and says "loading data...". Once it has though, navigating historical data will be just as quick as recent data as it will all be in the transient table.
If you refresh the screen or go back onto it from another screen, it clears out the transient table and starts again from the Current table data.

Hope my answer makes sense, makes the right assumptions ...
I would divide the news feed into two sections. The first is for incoming news - which would be powered with AJAX calls. It is constantly saying "What is new?" The second section is for older news, where the user can lazily load more news by scrolling down.
Newest News Items
The important point is to make note of the maximum news feed id on your page. Let's imagine that is 10000. When the user loaded the page, news feed id 10000 was the latest news item.
When the new section is updated with AJAX, we simply ask, "What is newer than id 10000?" and we load those items onto the page. After we load them, we also increment the id on the page. For example, if we start with id 10000 and we load five new news items, the new id would be 10005. The next call would ask, "What is newer than 10005?"
Older News Items
The older section would keep track of the oldest news item on the page. Let's imagine they scroll back for a weeks worth of news. The minimum news item id would be 9000. When they want to scroll back further, we simply ask, "What is older than 9000?"
The idea then is to maintain on the page the maximum news item id and the minimum news item id and then keep loading from that reference point.

Need design suggestion for storing data for budget keeping application

I'm writing an application that I will use to keep up with my monthly budget. This will be a C# .NET 4.0 Winforms application.
Essentially my budget will be a matrix of data if you look at it visually. The columns are the "dates" at which that budget item will be spent. For example, I have 1 column for every Friday in the month. The Y axis is the name of the budget item (Car payment, house payment, eating out, etc). There are also categories, which are used to group the budget item names that are similar. For example, a category called "Housing" would have budget items called Mortgage, Rent, Electricity, Home Insurance, etc.
I need a good way to store this data from a code design perspective. Basically I've thought of two approaches:
One, I can have a "BudgetItem" class that has a "Category", "Value", and "Date". I would have a linear list of these items and each time I wanted to find a value by either date or category, I iterate this list in some form or fashion to find the value. I could probably use LINQ for this.
Second, I could use a 2D array which is indexed first by column (date) and second by row. I'd have to maintain categories and budget item names in a separate list and join the data together when I do my lookups somehow.
What is the best way to store this data in code? I'm leaning more towards the first solution but I wanted to see what you guys think. Later on when I implement my data persistence, I want to be able to persist this data to SQL server OR to an XML file (one file per monthly budget).

While your first attempt looks nicer, obviusly the second could be faster (depends on how you implement it). However when we are talking about desktop applications which are not performance critical, your first idea is definitely better, expecially because will help you a lot talking about maintaining your code. Also remember that the entity framework could be really nice in this situation
Finally if you know how to works with XML, I think is really better for this type of project. A database is required only when you have a fair amount of tables, as you explained you will only have 2 tables (budgetitem and category), I don't think you need a database for such a simple thing

Is it good to store static data in db - C#?

I have this problem and I don't know what is the best solution for it.
I have table called Employees and there is column called LastWork, this column should only have custom values I choose for example:
value 1
value 2
and I want the user to select the value from ComboBox control so I have 2 ideas for it but I don't know what is the best for it.
A - add these value to Combobox as string in Items property and store them as string in DB.
B - create separate table in my db called for example 'LastWork' with 2 columns 'LastWorkID', 'LastWorkName' and insert my values in it, and then I can add binding source control and I can use data bound items to store the id as integer in my main table and show the LastWorkName for users.
I prefer to use the B method because in some forms I have DataGridView control with edit permission, and I want to display Combobox in it instead of Textbox to select from these custom values.
I hope you understood my questions.

Normally data normalization is a good thing, so I too would go with your option B.
By having a separate table and a foreign key relationship to it, you can enforce data integrity; easily get a list of all available (not just all selected) options; have a single place in which to change the text of an option (what if someone decides to call it "value one" instead of "value 1", for example?); and so on and so forth.
These might not be huge benefits in a small application and with only two possible options, but we all know that applications very often tend to grow in scope over time.

In a normalized database, your "option B" is usually the way to go because it eliminates duplicate data. It will potentially introduce an additional join into your queries when you need the name (and not just the ID), but it also allows you to rename lookup names easily without altering their underlying IDs.
For performance reasons, it's often a good idea to cache lookup values such as you describe in the business tier so that your lookup table is not hit over and over again (such as when building many rows of a grid).

I would always save them in the db. If you have to localize your app, this helps alot. Additonally, it let you to apply the referential integrity checks of the database.