a adduser Add a user to the system
addgroup Add a group to the system
alias Create an alias •
apropos Search Help manual pages (man -k)
apt-get Search for and install software packages (Debian/Ubuntu)
aptitude Search for and install software packages (Debian/Ubuntu)
aspell Spell Checker
awk Find and Replace text, database sort/validate/index
b
basename Strip directory and suffix from filenames
bash GNU Bourne-Again SHell
bc Arbitrary precision calculator language
bg Send to background
break Exit from a loop •
builtin Run a shell builtin
bzip2 Compress or decompress named file(s)
c
cal Display a calendar
case Conditionally perform a command
cat Display the contents of a file
cd Change Directory
cfdisk Partition table manipulator for Linux
chgrp Change group ownership
chmod Change access permissions
chown Change file owner and group
chroot Run a command with a different root directory
chkconfig System services (runlevel)
cksum Print CRC checksum and byte counts
clear Clear terminal screen
cmp Compare two files
comm Compare two sorted files line by line
command Run a command - ignoring shell functions •
continue Resume the next iteration of a loop •
cp Copy one or more files to another location
cron Daemon to execute scheduled commands
crontab Schedule a command to run at a later time
csplit Split a file into context-determined pieces
cut Divide a file into several parts
d
date Display or change the date & time
dc Desk Calculator
dd Convert and copy a file, write disk headers, boot records
ddrescue Data recovery tool
declare Declare variables and give them attributes •
df Display free disk space
diff Display the differences between two files
diff3 Show differences among three files
dig DNS lookup
dir Briefly list directory contents
dircolors Colour setup for `ls'
dirname Convert a full pathname to just a path
dirs Display list of remembered directories
dmesg Print kernel & driver messages
du Estimate file space usage
e
echo Display message on screen •
egrep Search file(s) for lines that match an extended expression
eject Eject removable media
enable Enable and disable builtin shell commands •
env Environment variables
ethtool Ethernet card settings
eval Evaluate several commands/arguments
exec Execute a command
exit Exit the shell
expect Automate arbitrary applications accessed over a terminal
expand Convert tabs to spaces
export Set an environment variable
expr Evaluate expressions
f
false Do nothing, unsuccessfully
fdformat Low-level format a floppy disk
fdisk Partition table manipulator for Linux
fg Send job to foreground
fgrep Search file(s) for lines that match a fixed string
file Determine file type
find Search for files that meet a desired criteria
fmt Reformat paragraph text
fold Wrap text to fit a specified width.
for Expand words, and execute commands
format Format disks or tapes
free Display memory usage
fsck File system consistency check and repair
ftp File Transfer Protocol
function Define Function Macros
fuser Identify/kill the process that is accessing a file
g
gawk Find and Replace text within file(s)
getopts Parse positional parameters
grep Search file(s) for lines that match a given pattern
groups Print group names a user is in
gzip Compress or decompress named file(s)
h
hash Remember the full pathname of a name argument
head Output the first part of file(s)
help Display help for a built-in command •
history Command History
hostname Print or set system name
i
id Print user and group id's
if Conditionally perform a command
ifconfig Configure a network interface
ifdown Stop a network interface
ifup Start a network interface up
import Capture an X server screen and save the image to file
install Copy files and set attributes
j
jobs List active jobs •
join Join lines on a common field
k
kill Stop a process from running
killall Kill processes by name
l
less Display output one screen at a time
let Perform arithmetic on shell variables •
ln Make links between files
local Create variables •
locate Find files
logname Print current login name
logout Exit a login shell •
look Display lines beginning with a given string
lpc Line printer control program
lpr Off line print
lprint Print a file
lprintd Abort a print job
lprintq List the print queue
lprm Remove jobs from the print queue
ls List information about file(s)
lsof List open files
m
make Recompile a group of programs
man Help manual
mkdir Create new folder(s)
mkfifo Make FIFOs (named pipes)
mkisofs Create an hybrid ISO9660/JOLIET/HFS filesystem
mknod Make block or character special files
more Display output one screen at a time
mount Mount a file system
mtools Manipulate MS-DOS files
mtr Network diagnostics (traceroute/ping)
mv Move or rename files or directories
mmv Mass Move and rename (files)
n
netstat Networking information
nice Set the priority of a command or job
nl Number lines and write files
nohup Run a command immune to hangups
notify-send Send desktop notifications
nslookup Query Internet name servers interactively
o
open Open a file in its default application
op Operator access
p
passwd Modify a user password
paste Merge lines of files
pathchk Check file name portability
ping Test a network connection
pkill Stop processes from running
popd Restore the previous value of the current directory
pr Prepare files for printing
printcap Printer capability database
printenv Print environment variables
printf Format and print data •
ps Process status
pushd Save and then change the current directory
pwd Print Working Directory
q
quota Display disk usage and limits
quotacheck Scan a file system for disk usage
quotactl Set disk quotas
r
ram ram disk device
rcp Copy files between two machines
read Read a line from standard input •
readarray Read from stdin into an array variable •
readonly Mark variables/functions as readonly
reboot Reboot the system
rename Rename files
renice Alter priority of running processes
remsync Synchronize remote files via email
return Exit a shell function
rev Reverse lines of a file
rm Remove files
rmdir Remove folder(s)
rsync Remote file copy (Synchronize file trees)
s
screen Multiplex terminal, run remote shells via ssh
scp Secure copy (remote file copy)
sdiff Merge two files interactively
sed Stream Editor
select Accept keyboard input
seq Print numeric sequences
set Manipulate shell variables and functions
sftp Secure File Transfer Program
shift Shift positional parameters
shopt Shell Options
shutdown Shutdown or restart linux
sleep Delay for a specified time
slocate Find files
sort Sort text files
source Run commands from a file `.'
split Split a file into fixed-size pieces
ssh Secure Shell client (remote login program)
strace Trace system calls and signals
su Substitute user identity
sudo Execute a command as another user
sum Print a checksum for a file
suspend Suspend execution of this shell •
symlink Make a new name for a file
sync Synchronize data on disk with memory
t
tail Output the last part of files
tar Tape ARchiver
tee Redirect output to multiple files
test Evaluate a conditional expression
time Measure Program running time
times User and system times
touch Change file timestamps
top List processes running on the system
traceroute Trace Route to Host
trap Run a command when a signal is set(bourne)
tr Translate, squeeze, and/or delete characters
true Do nothing, successfully
tsort Topological sort
tty Print filename of terminal on stdin
type Describe a command •
u
ulimit Limit user resources •
umask Users file creation mask
umount Unmount a device
unalias Remove an alias •
uname Print system information
unexpand Convert spaces to tabs
uniq Uniquify files
units Convert units from one scale to another
unset Remove variable or function names
unshar Unpack shell archive scripts
until Execute commands (until error)
useradd Create new user account
usermod Modify user account
users List users currently logged in
uuencode Encode a binary file
uudecode Decode a file created by uuencode
v
v Verbosely list directory contents (`ls -l -b')
vdir Verbosely list directory contents (`ls -l -b')
vi Text Editor
vmstat Report virtual memory statistics
w
watch Execute/display a program periodically
wc Print byte, word, and line counts
whereis Search the user's $path, man pages and source files for a program
which Search the user's $path for a program file
while Execute commands
who Print all usernames currently logged in
whoami Print the current user id and name (`id -un')
Wget Retrieve web pages or files via HTTP, HTTPS or FTP
write Send a message to another user
x
xargs Execute utility, passing constructed argument list(s)
xdg-open Open a file or URL in the user's preferred application.
yes Print a string until interrupted
. Run a command script in the current shell
### Comment / Remark
access()Used to check the accessibility of files
int
Access(pathname, access_mode)
Char* pathname;
int access-mode;
The access modes are.
04 read
02 write
01 execute (search)
00 checks existence of a file
& operator
execute a command as a background process.
banner
prints the specified string in large letters. Each argument may be upto 10 characters long.
break
is used to break out of a loop. It does not exit from the program.
Cal
Produces a calender of the current month as standard output. The month (1-12) and year (1-9999) must be specified in full numeric format.
Cal [[ month] year]
Calendar
Displays contents of the calendar file
case operator
The case operator is used to validate multiple conditions.
Case $string in
Pattern 1)
Command list;;
Command list;;
Pattern 3)
Command list;;
easc
cat
(for concatenate) command is used to display the contents of a file. Used without arguments it takes input from standard input <Dtrl d> is used to terminate input.
cat [filename(s)]
cat > [filename]
Data can be appended to a file using >>
Some of the available options are :
Cat [-options] filename(S)
-s silent about files that
cannot be accessed
-v enables display of non printinging characters (except tabs, new lines, form-
feeds)
-t when used with –v, it causes tabs to be printed as ^I’s
-e when used with –v, it causes $ to be printed at the end of each line
The –t and –e options are ignored if the –v options is not specified.
cd
Used to change directories
chgrp
Changes the group that owns a file.
Chgrp [grou –id] [filename]
chmod
Allows file permissions to be changed for each user. File permissions can be changed only by the owner (s).
Chmod [+/-][rwx] [ugo] [filename]
chown
Used to change the owner of a file.
The command takes a file(s) as source files and the login id of another user as the target.
Chown [user-id] [filename]
cmp
The cmp command compares two files (text or binary) byte-by-byte and displays the first occurrence where the files differ.
Cmp [filename1] [filename2] -1 gives a long listing
comm.
The comm command compares two sorted files and displays the instances that are common. The display is separated into 3 columns.
Comm. filename1 filename2
first displays what occurs in first files but not in the second
second displays what occurs in second file but not in first
third displays what is common in both files
continue statement
The rest of the commands in the loop are ignored. It moves out of the loop and moves on the next cycle.
cp
The cp (copy) command is used to copy a file.
Cp [filename1] [filename2]
cpio(copy input/output)
Utility program used to take backups.
Cpio operates in three modes:
-o output
-i input
-p pass
creat()
the system call creates a new file or prepares to rewrite an existing file. The file pointer is set to the beginning of file.
#include<sys/tyes.h>
#include<sys/stat.h>
int creat(path, mode)
char *path;
int mode;
cut
used to cut out parts of a file. It takes filenames as command line arguments or input from standard input. The command can cut columns as well as fields in a file. It however does not delete the selected parts of the file.
Cut [-ef] [column/fie,d] filename
Cut-d “:” –f1,2,3 filename
Where –d indicates a delimiter specified within “:”
df
used to find the number of free blocks available for all the mounted file systems.
#/etc/df [filesystem]
diff
the diff command compares text files. It gives an index of all the lines that differ in the two files along with the line numbers. It also displays what needs to be changed.
Diff filename1 filename2
echo
The echo command echoes arguments on the command line.
echo [arguments]
env
Displays the permanent environment variables associated with a user’s login id
exit command
Used to stop the execution of a shell script.
expr command
Expr (command) command is used for numeric computation.
The operators + (add), -(subtract), *(multiplu), /(divide), (remainder) are allowed. Calculation are performed in order of normal numeric precedence.
find
The find command searches through directories for files that match the specified criteria. It can take full pathnames and relative pathnames on the command line.
To display the output on screen the –print option must be specified
for operator
The for operator may be used in looping constructs where there is repetitive execution of a section of the shell program.
For var in vall val2 val3 val4;
Do commnds; done
fsck
Used to check the file system and repair damaged files. The command takes a device name as an argument
# /etc/fsck /dev/file-system-to-be-checked.
grave operator
Used to store the standard the output of a command in an enviroment variable. (‘)
grep
The grep (global regular expression and print) command can be used as a filter to search for strings in files. The pattern may be either a fixed character string or a regular expression.
Grep “string” filename(s)
HOME
User’s home directory
if operator
The if operator allows conditional operator
If expression; then commands; fi
if … then…else… fi
$ if; then
commands
efile; then
commands
fi
kill
used to stop background processes
In
used to link files. A duplicate of a file is created with another name
LOGNAME
displays user’s login name
ls
Lists the files in the current directory
Some of the available options are:
-l gives a long listing
-a displays all file{including hidden files
lp
used to print data on the line printer.
Lp [options] filename(s)
mesg
The mesg command controls messages received on a terminal.
-n does not allow messages to be displayed on screen
-y allows messages to be displayed on screen
mkdir
used to create directories
more
The more command is used to dispay data one screenful at a time.
More [filename]
mv
Mv (move) moves a file from one directory to another or simply changes filenames. The command takes filename and pathnames as source names and a filename or exiting directory as target names.
mv [source-file] [target-file]
news
The news command allows a user to read news items published by the system administrator.
ni
Displays the contents of a file with line numbers
passwd
Changes the password
paste
The paste command joins lines from two files and displays the output. It can take a number of filenames as command line arguments.
paste file1 file2
PATH
The directories that the system searches to find commands
pg
Used to display data one page (screenful) at a time. The command can take a number of filenames as arguments.
Pg [option] [filename] [filename2]…..
pipe
Operator (1) takes the output of one commands as input of another command.
ps
Gives information about all the active processes.
PS1
The system prompt
pwd
(print working directory) displays the current directory.
rm
The rm (remove) command is used to delete files from a directory. A number of files may be deleted simultaneously. A file(s) once deleted cannot be retrieved.
rm [filename 1] [filename 2]…
sift command
Using shift $1becomes the source string and other arguments are shifted. $2 is shifted to $1,$3to $2 and so on.
Sleep
The sleep command is used to suspend the execution of a shell script for the specified time. This is usually in seconds.
sort
Sort is a utility program that can be used to sort text files in numeric or alphabetical order
Sort [filename]
split
Used to split large file into smaller files
Split-n filename
Split can take a second filename on the command line.
su
Used to switch to superuser or any other user.
sync
Used to copy data in buffers to files
system0
Used to run a UNIX command from within a C program
tail
The tail command may be used to view the end of a file.
Tail [filename]
tar
Used to save and restore files to tapes or other removable media.
Tar [function[modifier]] [filename(s)]
tee
output that is being redirected to a file can also be viewed on standard output.
test command
It compares strings and numeric values.
The test command has two forms : test command itself If test ${variable} = value then
Do commands else do commands
File
The test commands also uses special operators [ ]. These are operators following the of are interpreted by the shell as different from wildcard characters.
Of [ -f ${variable} ]
Then
Do commands
Elif
[ -d ${variable} ]
then
do commands
else
do commands
fi
many different tests are possible for files. Comparing numbers, character strings, values of environment variables.
time
Used to display the execution time of a program or a command. Time is reported in seconds.
Time filename values
tr
The tr command is used to translate characters.
tr [-option] [string1 [string2]]
tty
Displays the terminal pathname
umask
Used to specify default permissions while creating files.
uniq
The uniq command is used to display the uniq(ue) lines in a sorted file.
Sort filename uniq
until
The operator executes the commands within a loop as long as the test condition is false.
wall
Used to send a message to all users logged in.
# /etc/wall message
wait
the command halts the execution of a script until all child processes, executed as background processes, are completed.
wc
The wc command can be used to count the number of lines, words and characters in a fine.
wc [filename(s)]
The available options are:
wc –[options] [filename]
-1
-w
-c
while operator
the while operator repeatedly performs an operation until the test condition proves false.
$ while
Ø do
commands
Ø done
who
displays information about all the users currently logged onto the system. The user name, terminal number and the date and time that each user logged onto the system.
The syntax of the who command is who [options]
write
The write command allows inter-user communication. A user can send messages by addressing the other user’s terminal or login id.
write user-name [terminal number]
Basic shell scripting questions
- How do you find out what’s your shell? - echo $SHELL
- What’s the command to find out today’s date? - date
- What’s the command to find out users on the system? - who
- How do you find out the current directory you’re in? - pwd
- How do you remove a file? - rm
- How do you remove a - rm -rf
- How do you find out your own username? - whoami
- How do you send a mail message to somebody? - mail somebody@techinterviews.com -s ‘Your subject’ -c ‘cc@techinterviews.com‘
- How do you count words, lines and characters in a file? - wc
- How do you search for a string inside a given file? - grep string filename
- How do you search for a string inside a directory? - grep string *
- How do you search for a string in a directory with the subdirectories recursed? - grep -r string *
- What are PIDs? - They are process IDs given to processes. A PID can vary from 0 to 65535.
- How do you list currently running process? - ps
- How do you stop a process? - kill pid
- How do you find out about all running processes? - ps -ag
- How do you stop all the processes, except the shell window? - kill 0
- How do you fire a process in the background? - ./process-name &
- How do you refer to the arguments passed to a shell script? - $1, $2 and so on. $0 is your script name.
- What’s the conditional statement in shell scripting? - if {condition} then … fi
- How do you do number comparison in shell scripts? - -eq, -ne, -lt, -le, -gt, -ge
- How do you test for file properties in shell scripts? - -s filename tells you if the file is not empty, -f filename tells you whether the argument is a file, and not a directory, -d filename tests if the argument is a directory, and not a file, -w filename tests for writeability, -r filename tests for readability, -x filename tests for executability
- How do you do Boolean logic operators in shell scripting? - ! tests for logical not, -a tests for logical and, and -o tests for logical or.
- How do you find out the number of arguments passed to the shell script? - $#
- What’s a way to do multilevel if-else’s in shell scripting? - if {condition} then {statement} elif {condition} {statement} fi
- How do you write a for loop in shell? - for {variable name} in {list} do {statement} done
- How do you write a while loop in shell? - while {condition} do {statement} done
- How does a case statement look in shell scripts? - case {variable} in {possible-value-1}) {statement};; {possible-value-2}) {statement};; esac
- How do you read keyboard input in shell scripts? - read {variable-name}
- How do you define a function in a shell script? - function-name() { #some code here return }
- How does getopts command work? - The parameters to your script can be passed as -n 15 -x 20. Inside the script, you can iterate through the getopts array as while getopts n:x option, and the variable $option contains the value of the entered option.
Linux command line Q&A
By admin | July 15, 2008
- You need to see the last fifteen lines of the files dog, cat and horse. What command should you use?
tail -15 dog cat horse
The tail utility displays the end of a file. The -15 tells tail to display the last fifteen lines of each specified file. - Who owns the data dictionary?
The SYS user owns the data dictionary. The SYS and SYSTEM users are created when the database is created.
- You routinely compress old log files. You now need to examine a log from two months ago. In order to view its contents without first having to decompress it, use the _________ utility.
zcat
The zcat utility allows you to examine the contents of a compressed file much the same way that cat displays a file. - You suspect that you have two commands with the same name as the command is not producing the expected results. What command can you use to determine the location of the command being run?
which
The which command searches your path until it finds a command that matches the command you are looking for and displays its full path. - You locate a command in the /bin directory but do not know what it does. What command can you use to determine its purpose.
whatis
The whatis command displays a summary line from the man page for the specified command. - You wish to create a link to the /data directory in bob’s home directory so you issue the command ln /data /home/bob/datalink but the command fails. What option should you use in this command line to be successful.
Use the -F option
In order to create a link to a directory you must use the -F option. - When you issue the command ls -l, the first character of the resulting display represents the file’s ___________.
type
The first character of the permission block designates the type of file that is being displayed. - What utility can you use to show a dynamic listing of running processes? __________
top
The top utility shows a listing of all running processes that is dynamically updated. - Where is standard output usually directed?
to the screen or display
By default, your shell directs standard output to your screen or display. - You wish to restore the file memo.ben which was backed up in the tarfile MyBackup.tar. What command should you type?
tar xf MyBackup.tar memo.ben
This command uses the x switch to extract a file. Here the file memo.ben will be restored from the tarfile MyBackup.tar. - You need to view the contents of the tarfile called MyBackup.tar. What command would you use?
tar tf MyBackup.tar
The t switch tells tar to display the contents and the f modifier specifies which file to examine. - You want to create a compressed backup of the users’ home directories. What utility should you use?
tar
You can use the z modifier with tar to compress your archive at the same time as creating it. - What daemon is responsible for tracking events on your system?
syslogd
The syslogd daemon is responsible for tracking system information and saving it to specified log files. - You have a file called phonenos that is almost 4,000 lines long. What text filter can you use to split it into four pieces each 1,000 lines long?
split
The split text filter will divide files into equally sized pieces. The default length of each piece is 1,000 lines. - You would like to temporarily change your command line editor to be vi. What command should you type to change it?
set -o vi
The set command is used to assign environment variables. In this case, you are instructing your shell to assign vi as your command line editor. However, once you log off and log back in you will return to the previously defined command line editor. - What account is created when you install Linux?
root
Whenever you install Linux, only one user account is created. This is the superuser account also known as root. - What command should you use to check the number of files and disk space used and each user’s defined quotas?
repquota
The repquota command is used to get a report on the status of the quotas you have set including the amount of allocated space and amount of used space.
A pretty funny story - someone was offered a test of basic Linux questions for a junior network administrator position, and figured out that the best way to impress the future employer with good answers is to post the list on UbuntuForums. Good idea, right? That’s where all the Linux experts hang out. Unfortunately, that’s where the employer hung out as well:
It didn’t seem to me that I was asking too much for people to use mailing lists, forums, IRC whatever to compile the answers themselves. I actually expected to see some questions show up on forums but I didn’t expect someone to paste the entire thing and expect the forum users to do all the work that would qualify you for an interview. I think at this point you could save us all some time and not turn the answers back in, I already have the information I need on your answers.
Anyway, the list has been made public, so enjoy:
- Give an example of set of shell commands that will give you the number of files in a directory
- How do you tell what process has a TCP port open in Linux
- On a Red Hat Linux Variant how do you control whether a service starts when the system boots
- How do you tell the amount of free disk space left on a volume
- Give an example of a set of shell commands to tell how many times “bob� has logged on to the system this month
- Give an example of a recursively copying a directory from one location to another.
- How do you modify the IP and Net mask of a system running a Red Hat Variant of Linux
- Give an example of a set of shell commands that will give you the number of “httpd� processes running on a Linux box.
- On CentOS or Fedora based system using the package management application, how do you tell what package provided the file “libnss_ldap.so�
- What is the difference between VTP client, server, and transparent
- What is the maximum length of CAT6
- How does one set up a layer two link to share VLANs
- How does one implement redundant links at Layer 2
- What is the difference between a hub, switch, and a router? What are the security advantages of switch vs. hub?
- Show an example of using telnet to learn the headers of an http server.
- In what OSI layer does PPP exist
- What’s the difference between TCP and UDP
- Given a DNS server that has just started (with an empty cache) and host contacting this DNS server (using it’s OS setting) to learn an address for google.com, list the steps the DNS server will take to learn it with IP addresses (each step will have multiple possible IP addresses – you need choose only one per step).
- Why are layer 2 loops bad, and what protocol was designed to prevent them
- Given a radius server at 10.0.0.2 and a shared key of ‘abc123′ show the IOS commands necessary to authenticate switch users against the radius server, while still allowing the use of local username / password pairs
- What are two methods of retrieving SQL?
- What cursor type do you use to retrieve multiple recordsets?
- What is the difference between a "where" clause and a "having" clause? - "Where" is a kind of restiriction statement. You use where clause to restrict all the data from DB.Where clause is using before result retrieving. But Having clause is using after retrieving the data.Having clause is a kind of filtering command.
- What is the basic form of a SQL statement to read data out of a table? The basic form to read data out of table is ‘SELECT * FROM table_name; ‘ An answer: ‘SELECT * FROM table_name WHERE xyz= ‘whatever’;’ cannot be called basic form because of WHERE clause.
- What structure can you implement for the database to speed up table reads? - Follow the rules of DB tuning we have to: 1] properly use indexes ( different types of indexes) 2] properly locate different DB objects across different tablespaces, files and so on.3] create a special space (tablespace) to locate some of the data with special datatype ( for example CLOB, LOB and …)
- What are the tradeoffs with having indexes? - 1. Faster selects, slower updates. 2. Extra storage space to store indexes. Updates are slower because in addition to updating the table you have to update the index.
- What is a "join"? - ‘join’ used to connect two or more tables logically with or without common field.
- What is "normalization"? "Denormalization"? Why do you sometimes want to denormalize? - Normalizing data means eliminating redundant information from a table and organizing the data so that future changes to the table are easier. Denormalization means allowing redundancy in a table. The main benefit of denormalization is improved performance with simplified data retrieval and manipulation. This is done by reduction in the number of joins needed for data processing.
- What is a "constraint"? - A constraint allows you to apply simple referential integrity checks to a table. There are four primary types of constraints that are currently supported by SQL Server: PRIMARY/UNIQUE - enforces uniqueness of a particular table column. DEFAULT - specifies a default value for a column in case an insert operation does not provide one. FOREIGN KEY - validates that every value in a column exists in a column of another table. CHECK - checks that every value stored in a column is in some specified list. Each type of constraint performs a specific type of action. Default is not a constraint. NOT NULL is one more constraint which does not allow values in the specific column to be null. And also it the only constraint which is not a table level constraint.
- What types of index data structures can you have? - An index helps to faster search values in tables. The three most commonly used index-types are: - B-Tree: builds a tree of possible values with a list of row IDs that have the leaf value. Needs a lot of space and is the default index type for most databases. - Bitmap: string of bits for each possible value of the column. Each bit string has one bit for each row. Needs only few space and is very fast.(however, domain of value cannot be large, e.g. SEX(m,f); degree(BS,MS,PHD) - Hash: A hashing algorithm is used to assign a set of characters to represent a text string such as a composite of keys or partial keys, and compresses the underlying data. Takes longer to build and is supported by relatively few databases.
- What is a "primary key"? - A PRIMARY INDEX or PRIMARY KEY is something which comes mainly from
database theory. From its behavior is almost the same as an UNIQUE INDEX, i.e. there may only be one of each value in this column. If you call such an INDEX PRIMARY instead of UNIQUE, you say something about
your table design, which I am not able to explain in few words. Primary Key is a type of a constraint enforcing uniqueness and data integrity for each row of a table. All columns participating in a primary key constraint must possess the NOT NULL property. - What is a "functional dependency"? How does it relate to database table design? - Functional dependency relates to how one object depends upon the other in the database. for example, procedure/function sp2 may be called by procedure sp1. Then we say that sp1 has functional dependency on sp2.
- What is a "trigger"? - Triggers are stored procedures created in order to enforce integrity rules in a database. A trigger is executed every time a data-modification operation occurs (i.e., insert, update or delete). Triggers are executed automatically on occurance of one of the data-modification operations. A trigger is a database object directly associated with a particular table. It fires whenever a specific statement/type of statement is issued against that table. The types of statements are insert,update,delete and query statements. Basically, trigger is a set of SQL statements A trigger is a solution to the restrictions of a constraint. For instance: 1.A database column cannot carry PSEUDO columns as criteria where a trigger can. 2. A database constraint cannot refer old and new values for a row where a trigger can.
- Why can a "group by" or "order by" clause be expensive to process? - Processing of "group by" or "order by" clause often requires creation of Temporary tables to process the results of the query. Which depending of the result set can be very expensive.
- What is "index covering" of a query? - Index covering means that "Data can be found only using indexes, without touching the tables"
- What types of join algorithms can you have?
- What is a SQL view? - An output of a query can be stored as a view. View acts like small table which meets our criterion. View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space. View is a good way to present data in a particular format if you use that query quite often. View can also be used to restrict users from accessing the tables directly.
