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My SQL Interview Questions and Answers


MySQL - What the Privilege System Does
The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.
Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

MySQL User Names and Passwords
There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:
User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the -u or --user options. This means that you can't make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn't have a password. MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters. MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine. MySQL encrypts passwords using a different algorithm than the one used during the Unix login process.
Note that even if the password is
stored 'scrambled', and knowing your 'scrambled' password is enough to be able to connect to the MySQL server!

Connecting to the MySQL Server
MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name, and your password. For example, the mysql client can be started like this (optional arguments are enclosed between `[' and `]'):

shell> mysql [-h host_name] [-u user_name] [-pyour_pass]

Alternate forms of the -h, -u, and -p options are --host=host_name, --user=user_name, and --password=your_pass. Note that there is no space between -p or --password= and the password following it.

NOTE: Specifying a password on the command line is not secure! Any user on your system may then find out your password by typing a command like: ps auxww.

mysql uses default values for connection parameters that are missing from the command line:

The default hostname is localhost.
The default user name is your Unix login name.
No password is supplied if -p is missing.
Thus, for a Unix user joe, the following commands are equivalent:

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:

You can specify connection parameters in the [client] section of the `.my.cnf' configuration file in your home directory. The relevant section of the file might look like this:
[client]
host=host_name
user=user_name
password=your_pass

You can specify connection parameters using environment variables. The host can be specified for mysql using MYSQL_HOST. The MySQL user name can be specified using USER (this is for Windows only). The password can be specified using MYSQL_PWD (but this is insecure; see the next section).

MySQL - Keeping Your Password Secure
It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:
Never give a normal user access to the mysql.user table. Knowing the encrypted password for a user makes it possible to login as this user. The passwords are only scrambled so that one shouldn't be able to see the real password you used (if you happen to use a similar password with your other applications).
Use a -pyour_pass or --password=your_pass option on the command line. This is convenient but insecure, because your password becomes visible to system status programs (such as ps) that may be invoked by other users to display command lines. (MySQL clients typically overwrite the command-line argument with zeroes during their initialization sequence, but there is still a brief interval during which the value is visible.)
Use a -p or --password option (with no your_pass value specified). In this case, the client program solicits the password from the terminal:
shell> mysql -u user_name -p
Enter password: ********

The `*' characters represent your password. It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs non-interactively, there is no opportunity to enter the password from the terminal. On some systems, you may even find that the first line of your script is read and interpreted (incorrectly) as your password!
Store your password in a configuration file. For example, you can list your password in the [client] section of the `.my.cnf' file in your home directory:
[client]
password=your_pass

If you store your password in `.my.cnf', the file should not be group or world readable or writable. Make sure the file's access mode is 400 or 600.
You can store your password in the MYSQL_PWD environment variable, but this method must be considered extremely insecure and should not be used. Some versions of ps include an option to display the environment of running processes; your password will be in plain sight for all to see if you set MYSQL_PWD. Even on systems without such a version of ps, it is unwise to assume there is no other method to observe process environments.
All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected `.my.cnf' file.

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