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Which data type should I use to handle nine-digit account numbers and why?

Which data type should I use to handle 9-digit account numbers and why?
varchar(9) or int or decimal or something else ?
I'm talking from a database perspective — and the DBMS is Informix.

TL;DR Use CHAR(9).
You have a number of options, most of them mentioned in the comments. The options have different trade-offs. They include:
CHAR(9). This uses 9 bytes of storage, but can store leading zeros and that can save on formatting in the applications. You can write a check constraint that ensures that the value always contains 9 digits. If you later need to use longer numbers, you can extend the type easily to CHAR(13) or CHAR(16) or whatever.
INTEGER. This uses 4 bytes of storage. If you need leading zeros, you will have to format them yourself. If you later need more digits, you will need to change the type to BIGINT.
SERIAL. This could be used on one table and would automatically generate new values when you insert a zero into the column. Cross-referencing tables would use the INTEGER type.
DECIMAL(9,0). This uses 5 bytes of storage, and does not store leading zeros so you will have to format them yourself. If you later need more digits, you can change the type to DECIMAL(13,0) or DECIMAL(16,0) or whatever.
BIGINT and BIGSERIAL. These are 8-byte integers that can take you to 16 digits without problem. You have to provide leading zeros yourself.
INT8 and SERIAL8 — do not use these types.
VARCHAR(9). Not really appropriate since the length is not variable. It would require 10 bytes on disk where 9 is sufficient.
LVARCHAR(9). This is even less appropriate than VARCHAR(9).
NCHAR(9). This could be used as essentially equivalent to CHAR(9), but if you're only going to store digits, you may as well use CHAR(9).
NVARCHAR(9). Not appropriate for the same reasons that VARCHAR(9) and NCHAR(9) are not appropriate.
MONEY(9,0). Basically equivalent to DECIMAL(9,0) but might attract currency symbols — it would be better to use DECIMAL(9,0).
Any other type is rather quickly inappropriate, unless you design an extended type that uses INTEGER for storage but provides a conversion function to CHAR(9) that adds the leading zeros.

Number representation in a common command?

I have a line of code in a piece of C# I'm analyzing.
`random.next(0xf4240, 0x98967f).ToString();'
I know the command line is generating a number between the specified ranges and returning it as a string. Whats a little odd to me is the '0xf' and the '0x#####f'
I looked up that the 0xf is supposed to return a nybbie but I'd realy like to get an idea of what the raw values would be. Any help would be great.
Thanks.

The prefix 0x is how you specify hexadecimal values in C# and a number of other languages. It's my belief that hexadecimal can only specify integer values, although I may be wrong.
In your case, 0xf4240 is the same as F4240 in hexadecimal, or a 1.000.000 in decimal. 0x98967f is the same as 9.999.999 in decimal.
One thing, this code was obviously obfuscated on purpose, which is baaaad. There seems to be no need to provide those values in hexadecimal.

The command line isn't generating anything - your C# application (which happens to output to the command line) is calculating a pseudo-random number between 1000000 and 9999999 (you are passing in the hex representations).

In C#, 0x is used as prefix to represent hexadecimal integer literals. See the spec
In your case, f4240 and 98967f are just two integers represented in hexadecimal system.
Update:
As #codesparkle has stated they represent 1000000 and 9999999 respectively

Storing stories in sql server 2008?

I am going to store stories in nvarchar(MAX) fields in SQL Server, but I know the stories will be much longer than MAX allows, so what approach should I take? Should I split the story across multiple rows or should I skip using a database and use text files?

I believe the confusion stems from a misunderstanding of terms here.
nvarchar(n) is a data type where n can be a number from 1-4000. The number n in this case has a max of 4000, which adds up to 8000 bytes (2 bytes per character).
nvarchar(MAX) is a different data type altogether - the keyword MAX is a literal, and it is not a synonym for any potential value of n in my example above. Fields of this type have a maximum length of 2^31-1 characters, or over 1 billion, which adds up to over 2 billion bytes (2 bytes per character).
The same principles apply to varchar(n) and varchar(MAX), except each character may only be 1 byte, in which case the number of characters that can be stored is double. Whether it is only 1 byte depends on the collation, as Martin Smith notes in a comment!

Store them in chapters.
This is not technical - it is pretty much impossible to have astory of 1 billion nvarchar characters (and nvarchar(max) is the "new" TEXT data type.
BUt loading and processing them will be painfull.
Store them as chapters and store a start / end page number for every chapter when it makes sense, so you can navigate a little easier.
Btw., you posted you thought it is 800 chars - that was NEVER trhe case. The limit would be 8000 bytes - if it would apply - and that would be 4000 chars unicode.

I'd probably suggest looking into document oriented databases for something like this.

Ok you could try storing as LONGTEXT (Mysql) or TEXT (MSSQL) (if you want to store objects I think you can use BLOB) datatype?

Reading a Cobol generated file

I’m currently on the task of writing a c# application, which is going sit between two existing apps. All I know about the second application is that it processes files generated by the first one. The first application is written in Cobol.
Steps:
1) Cobol application, writes some files and copies to a directory.
2) The second application picks these files up and processes them.
My C# app would sit between 1) an 2). It would have to pick up the file generated by 1), read it, modify it and save it, so that application 2)
wouldn’t know I have even been there.
I have a few problems.
First of all if I open a file generated by 1) in notepad, most of it is unreadable while other parts are.
If I read the file, modify it and save, I must save the file with the same notation used by the cobol application, so that app 2), doesn´t know I´ve been there.
I´ve tried reading the file this way, but it´s still unreadable:
Code:
string ss = #"filename";
using (FileStream fs = new FileStream(ss, FileMode.Open))
{
StreamReader sr = new StreamReader(fs);
string gg = sr.ReadToEnd();
}
Also if I find a way of making it readable (using some sort of encoding technique), I´m afraid that when I save the file again, I may change it´s original format.
Any thoughts? Suggestions?

To read the COBOL-genned file, you'll need to know:
First, you'll need the record layout (copybook) for the file. A COBOL record layout will look something like this:
01 PATIENT-TREATMENTS.
05 PATIENT-NAME PIC X(30).
05 PATIENT-SS-NUMBER PIC 9(9).
05 NUMBER-OF-TREATMENTS PIC 99 COMP-3.
05 TREATMENT-HISTORY OCCURS 0 TO 50 TIMES
DEPENDING ON NUMBER-OF-TREATMENTS
INDEXED BY TREATMENT-POINTER.
10 TREATMENT-DATE.
15 TREATMENT-DAY PIC 99.
15 TREATMENT-MONTH PIC 99.
15 TREATMENT-YEAR PIC 9(4).
10 TREATING-PHYSICIAN PIC X(30).
10 TREATMENT-CODE PIC 99.
You'll also need a copy of IBM's Principles of Operation (S/360, S370, z/OS, doesn't really matter for our purposes). Latest is available from IBM at
http://www-01.ibm.com/support/docview.wss?uid=isg2b9de5f05a9d57819852571c500428f9a (but you'll need an IBM account.
An older edition is available, gratis, at http://www.hack.org/mc/texts/principles-of-operation.pdf
Chapters 8 (Decimal Instructions) and 9 (Floating Point Overview and Support Instructions) are the interesting bits for our purposes.
Without that, you're pretty much lost.
Then, you need to understand COBOL data types. For instance:
PIC defines an alphameric formatted field (PIC 9(4), for example is 4 decimal digits, that might be filled with for space characters if missing). Pic 999V99 is 5 decimal digits, with an implied decimal point. So-on and so forthe.
BINARY is [usually] a signed fixed point binary integer. Usual sizes are halfword (2 octets) and fullword (4 octets).
COMP-1 is single precision floating point.
COMP-2 is double precision floating point.
If the datasource is an IBM mainframe, COMP-1 and COMP-2 likely won't be IEE floating point: it will be IBM's base-16 excess 64 floating point format. You'll need something like the S/370 Principles of Operation to help you understand it.
COMP-3 is 'packed decimal', of varying lengths. Packed decimal is a compact way of representing a decimal number. The declaration will look something like this: PIC S9999V99 COMP-3. This says that is it signed, consists of 6 decimal digits with an implied decimal point. Packed decimal represents each decimal digit as a nibble of an octet (hex values 0-9). The high-order digit is the upper nibble of the leftmost octet. The low nibble of the rightmost octet is a hex value A-F representing the sign. So the above PIC clause will require ceil( (6+1)/2 ) or 4 octets. the value -345.67, as represented by the above PIC clause will look like 0x0034567D. The actual sign value may vary (the default is C/positive, D/negative, but A, C, E and F are treated as positive, while only B and D are treated as negative). Again, see the S\370 Principles of Operation for details on the representation.
Related to COMP-3 is zoned decimal. This might be declared as `PIC S9999V99' (signed, 5 decimal digits, with an implied decimal point). Decimal digits, in EBCDIC, are the hex values 0xFO - 0xF9. 'Unpack' (mainframe machine instruction) takes a packed decimal field and turns in into a character field. The process is:
start with the rightmost octet. Invert it, so the sign nibble is on top and place it into the rightmost octet of the destination field.
Working from right to left (source and the target both), strip off each remaining nibble of the packed decimal field, and place it into the low nibble of the next available octet in the destination. Fill the high nibble with a hex F.
The operation ends when either the source or destination field is exhausted.
If the destination field is not exhausted, if it left-padded with zeroes by filling the remaining octets with decimal '0' (oxF0).
So our example value, -345.67, if stored with the default sign value (hex D), would get unpacked as 0xF0F0F0F3F4F5F6D7 ('0003456P', in EBDIC).
[There you go. There's a quiz later]
If the COBOL app lives on an IBM mainframe, has the file been converted from its native EBCDIC to ASCII? If not, you'll have to do the mapping your self (Hint: its not necessarily as straightforward as that might seem, since this might be a selective process -- only character fields get converted (COMP-1, COMP-2, COMP-3 and BINARY get excluded since they are a sequence of binary octets). Worse, there are multiple flavors of EBCDIC representations, due to the varying national implementations and varying print chains in use on different printers.
Oh...one last thing. The mainframe hardware tends to like different things aligned on halfword, word or doubleword boundaries, so the record layout may not map directly to the octets in the file as there may be padding octets inserted between fields to maintain the needed word alignment.
Good Luck.

I see from comments attached to your question that you are dealing with the “classic” COBOL batch file structure: Header record, detail records and trailer record.
This is probably bad news if you are responsible for creating the trailer record! The typical “trailer” record is used to identify the end-of-file and provides control information such as the number of records that precede it and various check sums and/or grand totals for “detail” records. In other words, you may need to read and summarize the entire file in order to create the trailer. Add to this the possibility that much of the data in the file is in Packed Decimal, Zoned Decimal or other COBOLish numeric data types, you could be in for a rough time.
You might want to question why you are adding trailer records to these files. Typically the “trailer” is produced by the same program or application that created the “detail” records. The trailer is supposed to act as a verification that the sending application/program wrote all of the data it was supposed to. The summary totals, counts etc. are used by the receiving application to verify that the detail records tally with the preceding details. This is supposed to serve as another verification that the sending application didn't muff up the data or that it was not corrupted en-route (no that wasn't a joke – but maybe it should be). When a "man in the middle" creates the trailers it kind of defeats the entire purpose of the exercise (no matter how flawed it might have been to begin with).

It would be useful to know which Cobol Dialect you are dealing with because there is
no single Cobol Format. Some Cobol Compilers (Micro Focus) put a "File Description" at the front of files (For Micro Focus VB / Indexed files).
Have a look at the RecordEditor (http://record-editor.sourceforge.net/). It has a File Wizard which could be very useful for you.
In the File Wizard set the file as Fixed-Width File (most common in Cobol). The program lets you try out different Record Lengths. When you get the correct record length, the Text fields should line up.
Latter on in the Wizard there is field search which can look for Binary, Comp-3, Text Fields.
There is some notes on using the RecordEditor's Wizard with an unknown file here
http://record-editor.sourceforge.net/Unkown.htm
Unless the file is coming from a Mainframe / AS400 it is unlikely to use EBCDIC (cp037 - Coded Page 37 is US EBCDIC), any text is most likely in Ascii.
The file probably contains Packed-Decimal (Comp3) and Binary-Integer data. Most Cobols
use Big-Endian (for Comp integers) even on Intel (little endian hardware).
One thing to remember with Cobol PIC s9(6)V99 comp is stored as a Binary Integer with x'0001' representing 0.01. So unless you have the Cobol definition you can not tell wether a binary 1 is 1 0.1, 0.01 etc

Using masked text box to validate input

Hi I am using the following mask string to ensure user enters a valid phone no
(99)-00000000
which should work fine for mobile as well as regional land line numbers in Australia
However I have encountered a problem that User can get away without entring all the digits
I understand that 9 represents an optional digit between 0 and 9 and 0 represents a required digit between 0 and 9
So how come if I dont enter all last eight mandatory digits the program still display results like
(03)-6474
(03)- 63799
(02)-1 38 390

Because it's simply filling in the blanks from left to right as you type. Even though the first two digits are optional, it fills them in like you've entered them with the first two numbers that you type.
If you dislike this behavior, you can easily subclass the MaskedTextBox control and customize how it handles keyboard input. Here are a couple of sample controls that cause the numbers to push in from right to left, as expected:
http://www.codeproject.com/KB/edit/Cool_Phone_Number_Box.aspx
http://www.dotnetheaven.com/Uploadfile/mgold/MaskedCurrencyTextBox02252006005553AM/MaskedCurrencyTextBox.aspx
(I think the second example does a better job of explaining exactly what changes are being made to the base control, plus it allows you to see most of the relevant code without downloading the sample project.)