SQL Server databases use two files - an MDF file, known as the primary database file, which contains the schema and data, and a LDF file, which contains the logs. See [wikipedia][1]. A database may also use secondary database file, which normally uses a .ndf extension.

As John S. indicates, these file extensions are purely convention - you can use whatever you want, although I can&#39;t think of a good reason to do that.

More info on MSDN [here][2] and in Beginning SQL Server 2005 Administation (Google Books) [here][3].


  [1]: http://en.wikipedia.org/wiki/Microsoft_SQL_Server#Data_storage
  [2]: http://msdn.microsoft.com/en-us/library/aa174545(SQL.80).aspx
  [3]: http://books.google.com/books?id=TVVLWLJtpIUC&amp;pg=PA17&amp;lpg=PA17&amp;dq=sql+server+primary+data+file+mdf&amp;source=bl&amp;ots=x63PpwvAVD&amp;sig=CyGAZCYtnkTqoggqxMJA9cSeIog&amp;hl=en&amp;ei=t0xpSqK2OoOIswO98_2WBQ&amp;sa=X&amp;oi=book_result&amp;ct=result&amp;resnum=3

Just to make this absolutely clear for all:

A .MDF file is “typically” a SQL Server data file however it is important to note that it does **NOT** have to be.

This is because .MDF is nothing more than a recommended/preferred notation but the extension itself does not actually dictate the file type.

To illustrate this, if someone wanted to create their primary data file with an extension of .gbn they could go ahead and do so without issue.

To qualify the preferred naming conventions:

 - .mdf - Primary database data file.
 - .ndf - Other database data files i.e.
   non Primary.
 - .ldf - Log data file.



We have a system that has some Bash scripts running besides Java code. Since we are trying to *test everything that could possibly break*, and those Bash scripts may break, we want to test them.

The problem is it is hard to test Bash scripts.

Is there a way or a best practice to test Bash scripts? Or should we quit using Bash scripts and look for alternative solutions that are testable?



Unit testing Bash scripts

In TSQL I could use something like `Select [table] from tablename` to select a column named &quot;table&quot;.

How do I do this for reserved words in oracle? 

Edit: I&#39;ve tried square braces, double quotes, single quotes, and backquotes, they don&#39;t work...

As a further clarification, I have a column which someone named comment.  As this is a reserved word oracle is chucking a wobbly trying to select with it, its failing when parsing the query.  I&#39;ve tried Select &quot;comment&quot; from tablename but it didn&#39;t work.  I&#39;ll check case and come back.

How do I escape a reserved word in Oracle?

Is this like an “embedded” database of sorts? A file containing a built in database?

What is an MDF file?

<p>Is this like an “embedded” database of sorts? A file containing a built in database?</p>


The following User History table contains **one record for every day a given user has accessed a website** (in a 24 hour UTC period). It has many thousands of records, but only one record per day per user. If the user has not accessed the website for that day, no record will be generated.

&lt;pre&gt;
Id      UserId   CreationDate
------  ------   ------------
750997      12   2009-07-07 18:42:20.723
750998      15   2009-07-07 18:42:20.927
751000      19   2009-07-07 18:42:22.283
&lt;/pre&gt;

What I&#39;m looking for is a SQL query on this table *with good performance*, that tells me which userids have accessed the website for (n) continuous days without missing a day.

In other words, **how many users have (n) records in this table with sequential (day-before, or day-after) dates**? If any day is missing from the sequence, the sequence is broken and should restart again at 1; we&#39;re looking for users who have achieved a continuous number of days here with no gaps.

Any resemblance between this query and [a particular Stack Overflow badge](https://stackoverflow.com/badges/71/woot-enthusiast) is purely coincidental, of course.. :)

SQL to determine minimum sequential days of access?

Which method provides the best performance when removing the time portion from a datetime field in SQL Server?

    a) select DATEADD(dd, DATEDIFF(dd, 0, getdate()), 0)
or

    b) select cast(convert(char(11), getdate(), 113) as datetime)

The second method does send a few more bytes either way but that might not be as important as the speed of the conversion.

Both also appear to be very fast, but there might be a difference in speed when dealing with hundreds-of-thousands or more rows?

Also, is it possible that there are even better methods to get rid of the time portion of a datetime in SQL?

Best approach to remove time part of datetime in SQL Server

I&#39;ve been learning Functions and Stored Procedure for quite a while but I don&#39;t know why and when I should use a function or a stored procedure. They look same to me, maybe because I am kinda newbie about that.

Can some one tell me why?

Function vs. Stored Procedure in SQL Server

What are the guidelines as to when to use `begin` and `end` blocks in SQL Server?  

Also, what exactly does the `Go` keyword do?

Use of Begin / End Blocks and the Go keyword in SQL Server?

Why can&#39;t I create a constraint on an `nvarchar(max)` column? SQL Server will not allow me to put a unique constraint on it. But, it allows me to create a unique constraint on an `nvarchar(100)` column.

Both of these columns are `NOT NULL`. Is there any reason I can&#39;t add a constraint to the `nvarchar(max)` column?

Why can&#39;t I put a constraint on nvarchar(max)?

I&#39;ve been messing around with Django and the Django ORM at home, and I&#39;ve got to say, I feel it is one of the best out there in terms of ease of use.

However, I was wondering if it was possible to use it in &quot;reverse&quot;.   

Basically what I would like to do is generate Django models from an existing database schema (from a project that doesn&#39;t use django and is pretty old).

Is this possible?

**Update:** the database in question is Oracle

Is it possible to generate django models from the database?

I&#39;m interested in hearing about design strategies you have used with **non-relational &quot;nosql&quot; databases** - that is, the (mostly new) class of data stores that don&#39;t use traditional relational design or SQL (such as Hypertable, CouchDB, SimpleDB, Google App Engine datastore, Voldemort, Cassandra, SQL Data Services, etc.). They&#39;re also often referred to as &quot;key/value stores&quot;, and at base they act like giant distributed persistent hash tables.

Specifically, I want to learn about the differences in *conceptual data design* with these new databases. What&#39;s easier, what&#39;s harder, what can&#39;t be done at all?

 - Have you come up with alternate designs that work much better in the non-relational world?

 - Have you hit your head against anything that seems impossible? 

 - Have you bridged the gap with any design patterns, e.g. to translate from one to the other? 

 - Do you even do explicit data models at all now (e.g. in UML) or have you chucked them entirely in favor of semi-structured / document-oriented data blobs?

 - Do you miss any of the major extra services that RDBMSes provide, like relational integrity, arbitrarily complex transaction support, triggers, etc?

I come from a SQL relational DB background, so normalization is in my blood. That said, I get the advantages of non-relational databases for simplicity and scaling, and my gut tells me that there has to be a richer overlap of design capabilities. What have you done?

FYI, there have been StackOverflow discussions on similar topics here: 

 - [the next generation of databases][1]
 - [changing schemas to work with Google App Engine][2]
 - [choosing a document-oriented database][3]


  [1]: https://stackoverflow.com/questions/282783/the-next-gen-databases
  [2]: https://stackoverflow.com/questions/731147/what-changes-do-i-need-for-my-tables-to-work-on-appengines-bigtable
  [3]: https://stackoverflow.com/questions/337344/pros-cons-of-document-based-database-vs-relational-database

Non-Relational Database Design

Can I add a UNIQUE constraint to a PostgreSQL table, after it&#39;s already created?

When storing latitude or longitude data in an ANSI SQL compliant database, what datatype would be most appropriate? Should `float` be used, or `decimal`, or ...?

I&#39;m aware that Oracle, MySql, and SQL Server have added some special datatypes specifically for handling geo data, but I&#39;m interested in how you would store the information in a &quot;plain vanilla&quot; SQL database.

What datatype to use when storing latitude and longitude data in SQL databases?

Here is an extracted portion of my query, reflecting the `EMAIL_ADDRESS` column data type and property:

    EMAIL_ADDRESS CHARACTER VARYING(20) NOT NULL, 

However, [John Saunders][1] uses `VARYING(256)`.

This suggests me that I have not necessarily understood the VARYING correctly.

I understand it such that the length of an email address is 20 characters in my case, while 256 for Jodn.

Context in John&#39;s code

    CREATE TABLE so.&quot;User&quot;
      (
        USER_ID SERIAL NOT NULL,
        USER_NAME CHARACTER VARYING(50) NOT NULL,
        EMAIL_ADDRESS CHARACTER VARYING(256) NOT NULL, // Here
        HASHED_PASSWORD so.HashedPassword NOT NULL,
        OPEN_ID CHARACTER VARYING(512),                                                         
        A_MODERATOR BOOLEAN,
        LOGGED_IN BOOLEAN,
        HAS_BEEN_SENT_A_MODERATOR_MESSAGE BOOLEAN,
        CONSTRAINT User_PK PRIMARY KEY(USER_ID)
      );

I have never seen email addresses longer than 20 characters, used by ordinary people.

What is the optimal length for an email address in a database?

  [1]: https://stackoverflow.com/questions/1196873/to-prevent-the-use-of-duplicate-tags-in-a-database/1197192#1197192

What is the optimal length for an email address in a database?

I need to find the most recently modified file in a directory. 

I know I can loop through every file in a folder and compare `File.GetLastWriteTime`, but is there a better way to do this without looping?.


How to find the most recent file in a directory using .NET, and without looping?

I&#39;m receiving a string from an external process. I want to use that String to make a filename, and then write to that file. Here&#39;s my code snippet to do this:

        String s = ... // comes from external source
        File currentFile = new File(System.getProperty(&quot;user.home&quot;), s);
        PrintWriter currentWriter = new PrintWriter(currentFile);

If s contains an invalid character, such as &#39;/&#39; in a Unix-based OS, then a java.io.FileNotFoundException is (rightly) thrown.

How can I safely encode the String so that it can be used as a filename?

Edit: What I&#39;m hoping for is an API call that does this for me.

I can do this:

        String s = ... // comes from external source
        File currentFile = new File(System.getProperty(&quot;user.home&quot;), URLEncoder.encode(s, &quot;UTF-8&quot;));
        PrintWriter currentWriter = new PrintWriter(currentFile);

But I&#39;m not sure whether URLEncoder it is reliable for this purpose.


How can I safely encode a string in Java to use as a filename?

ASP.NET has four different types of file results:  

 - FileContentResult: Sends the contents of a binary file to the response.
 - FilePathResult: Sends the contents of a file to the response
 - FileResult: Returns binary output to write to the response 
 - FileStreamResult: Sends binary content to the response by using a Stream instance 

Those descriptions are take from MSDN and with the exception of the FileStreamResult the first three sound identical. So what is the difference between them?

What&#39;s the difference between the four File Results in ASP.NET MVC

Is it possible to get the filename of a file descriptor (Linux) in C?

Retrieve filename from file descriptor in C

I&#39;m trying to get the absolute path of certain files in a C# class.  `Server.MapPath` works great of course for ASPX and their code-behind pages, but that doesn&#39;t exist in another class file.  I tried `HostingEnvironment.MapPath()`, but that complains that the relative virtual path isn&#39;t allowed.  Any thoughts?

`System.Web` is already imported.

Content Type	Original Author	Original Content on Stackoverflow
Question	NewGuy	View Question on Stackoverflow
Solution 1 - Mdf	Jon Galloway	View Answer on Stackoverflow
Solution 2 - Mdf	John Sansom	View Answer on Stackoverflow

What is an MDF file?

Mdf Problem Overview

Mdf Solutions

Solution 1 - Mdf

Solution 2 - Mdf

How do I escape a reserved word in Oracle?

Unit testing Bash scripts

Attributions