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This is another example of a nested query where we have two SQL SELECT statements, one nested inside the other. This one is a little different from the simpler example in Listing 4-5. The WHERE condition in the inner query refers to part of the row being considered in the outer query, that is, e.MemberID = m.MemberID. I find the easiest way to interpret Listing 4-10 is with reference to Figure 4-4. The inner query is looking for a row in the Entry table with the same value for MemberID as the row under consideration in the Member table. If such a row or several such rows EXIST, then we are in business.

Standard file Standard directory Symbolic link (a shortcut to another file) Named pipe (a file that acts as a conduit for data between two programs) Socket (a file designed to send and receive data over a network) Character device (a hardware device driver, usually found in /dev) Block device (a hardware device driver, usually found in /dev)

On a Linux system such as Ubuntu, everything is treated as a file. Even a device like your hard disk is addressed by pointing to a file. Therefore, working with files is the most important task when administering Linux. In this section, you ll learn the basics of managing a file system. The following subjects are covered: Working with directories Working with files Viewing text files Creating empty files

As with Windows, programs are stored as files on your hard disk, just like standard data files. On Linux, program files need to be explicitly marked as being executable. This is indicated in the permission listing by an x. Therefore, if there s no x in a file s permissions, it s a good bet that the file in question isn t a program or script (although this isn t always true for various technical reasons). To make matters a little more confusing, if the entry in the list of files is a directory (indicated by a d), then the rules are different. In this case, an x indicates that the user can access that directory. If there s no x, the user s attempts to browse to that directory will be met with an access denied message. File permissions can be difficult to understand, so let s look at a few real-world examples. These examples assume that you re logged into Linux as the user ubuntu.

Here s the first example:

An important goal of this chapter is to learn about setting up the best file storage for your server. Because everything on Linux is available as a file, setting up file storage space is one of the most important tasks on a Linux server. This goes for programs and regular text files, but also for more advanced things such as devices. Your server is probably going to host lots of different applications that all create their own files: some will create a few huge files, and others will require the fastest possible access to no matter what files, and others may be something like a mail server, creating thousands and thousands of small files. Ubuntu Server provides you the flexibility to choose the best file system for all these different needs because many file systems are supported right out of the box. Before diving in to the details that are needed to

For those of you who are thinking that this seems like a complicated way to get a simple result, you are right (partly). The query in Listing 4-10 retrieves the same members as an inner join (on MemberID) between Member and Entry. However, what if we want those members who have NOT entered a tournament This requires only a tiny change to our relational calculus and SQL expressions. Instead of looking for members where a matching row in Entry exists, we now want those where a matching row does NOT exist. Adding the word NOT to Listing 4-10 gives us what we require, as shown in Listing 4-11.

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You know that this file is owned by user ubuntu because that username appears in the Owner column Also notice that the group Ubuntu has access to the file, although precisely how much depends on the permissions From left to right, the initial file permission character is a hyphen, which indicates that this is an ordinary file and has no special characteristics It s also not a directory After that is the first part of the permissions, rw- These are the permissions for the owner of the file, ubuntu You re logged in as that user, so this file belongs to you, and these permissions apply to you You can read and write to the file but not execute it.