0:01	foreign
0:07	welcome back in this presentation we are
0:10	going to see some solved questions in
0:12	the topic keys in rdbms let's step into
0:16	question number one the question is
0:18	which of the following is correct about
0:21	keys in tbms and the options are given
0:24	option A it is mandatory in tables
0:28	option b it can be changed frequently
0:31	option C it can be a set of one or more
0:35	columns that uniquely identify a record
0:38	and option D all of the above I request
0:42	you to pause this video for a while and
0:44	think about the right answer
0:47	I hope you are done and the right answer
0:50	for this question is option C key can be
0:53	a set of one or more columns or
0:56	attributes that uniquely identify a
0:58	record why option A is incorrect because
1:02	we may have a table or a relation which
1:05	may contain a key attribute or may not
1:07	contain a key attribute and hence we
1:10	can't say that it is always mandatory in
1:12
the table or relations at the same time
1:15
the column or attribute that is chosen
1:17
as a key will not be changed frequently
1:20
only in a very rare situation it can be
1:23
changed but generally key attributes
1:25
cannot be changed frequently and that is
1:28
why for question number one option C is
1:31
the right answer
1:32
we are done with question number one
1:34
let's now move on to question number two
1:37
and the question is referring to the
1:40
below table a table is given which
1:42
attribute is appropriate to be a key
1:45
attribute so we are given with the table
1:47
employee which contains attributes ID
1:50
which means employee ID name of the
1:53
employee Department salary and City and
1:57
we are required to find which attribute
1:59
in this list of five attributes can be
2:02
appropriate to be a key attribute let's
2:05
see the options the options are option a
2:08
ID option b name option C salary and
2:12
option D none of the above I request you
2:15
to pause this video for a while and
2:16
think about the right answer
2:19
and the right answer for this question
2:21
is option a ID
2:23
every employee will have an ID and this
2:26
will be unique throughout the
2:28
organization and that's why we are
2:30
preferring ID as the key attribute why
2:32
we are not preferring name as a key
2:34
attribute because many employees may
2:36
have the same name the same time many
2:39
employees may have the same salary
2:40
structure also and that's why the column
2:43
or attribute ID can be appropriate to be
2:46
a key attribute
2:48
we are done with question number two
2:50
let's now move on to question number
2:52
three the question is referring to the
2:55
below schema which attribute or
2:58
attributes is or are appropriate for
3:01
being a key attribute and the schema is
3:04
the student relation which contains the
3:06
attributes name date of birth age City
3:10
and zip code
3:12
and what are all the options option a
3:15
name option b name and date of birth
3:18
option C name and City and option D none
3:22
of the above please pause this video for
3:24
a while and think about the right answer
3:27
let's analyze the options in this case
3:30
name alone cannot be a key attribute
3:32
because we know multiple students may
3:35
have the same name and that's why this a
3:37
cannot be a key attribute and coming to
3:40
name and date of birth this is a
3:42
composite key where multiple columns or
3:44
attributes combine together to form a
3:46
key in this case the composite key name
3:49
plus date of birth May uniquely identify
3:51
a tuple but we can't guarantee hundred
3:54
percent there are chances that student
3:56
name and the same date of birth may also
3:58
exist in the same database and coming to
4:01
option C name plus City again there may
4:04
be more than one student with the same
4:06
name from the same city as well so I
4:08
would say this is also not appropriate
4:10
for being a key and option D none of the
4:13
above and the right answer for this
4:15
question is option D none of the above
4:17
because name name plus date of birth and
4:21
name plus City cannot uniquely identify
4:23
a tuple but to some extent name and date
4:26
of birth May uniquely add identify but
4:29
it actually depends on the database
4:31
we are done with question number three
4:33
and let's move on to question number
4:35
four the question is the minimal super
4:37
key is called as Dash key and the
4:40
options are option a primary key option
4:44
b foreign key option C candidate key and
4:47
option D alternate key and the right
4:50
answer for this question is option C
4:52
candidate key because candidate key is
4:54
the minimal set of attributes that can
4:57
uniquely identify a tuple in simple
4:59
terms a candidate key is a minimal super
5:02
key with no repeated data a Super Key
5:04
may contain extraneous attributes but
5:07
this candidate key is the minimal Super
5:09
Key also a table can have multiple
5:11
candidate keys but exactly one primary
5:14
key why am I not choosing primary key as
5:16
the right answer because a primary key
5:19
itself is a candidate key and why
5:21
foreign key is not considered because
5:23
foreign key are used for referential
5:25
integrity where it involves two tables
5:28
and coming to the last option the
5:30
alternate key the candidate key other
5:32
than the primary key is called an
5:34
alternate key say for example we may
5:36
have five candidate keys and one of the
5:39
keys will be chosen as the primary key
5:40
and the rest of the keys are called as
5:43
alternate keys
5:44
we are done with question number four
5:46
let's move on to question number five
5:48
the question is the subset of super key
5:51
is a candidate key under what condition
5:54
and the options are option A any subset
5:58
is a Super Key option b all subsets are
6:02
super Keys option C no proper subset is
6:06
a Super Key and option D none of the
6:08
above
6:09
I request you to pause this video for a
6:12
while and think about the right answer
6:15
I hope you are done to find the answer
6:18
for this question I'm gonna bring in the
6:20
same example which we had seen in the
6:22
last presentation we know what is a
6:24
candidate key a candidate key is a
6:26
minimal Super Key and we know what is a
6:29
Super Key it is the superset the all
6:31
possible keys in a relation and this is
6:34
the superset actually let's take an
6:36
example here I am gonna take an example
6:38
called name and phone number which is a
6:41
Super Key and this is also a candidate
6:43
key we know how to derive candidate key
6:45
from the super keys if you are not sure
6:47
I request you to watch my previous
6:49
lecture about keys in rdbms so when we
6:53
talk about this key name and phone
6:55
number now this is actually a set and
6:58
when we say this is a set Motorola
7:00
subset we can derive from the set name
7:03
alone and phone number alone so when we
7:05
say name alone and phone number alone
7:07
are separate subsets and this is from
7:10
this set when we create the subset from
7:13
this set name phone number combo then
7:15
name alone and phone number alone are
7:17
separate subsets I mean name alone is
7:20
one subset phone number alone in another
7:22
subset but actually name alone and phone
7:25
number alone are not keys at all and
7:28
that's why option A is invalid any
7:30
subset is a Super Key is not correct and
7:33
coming to option b all subsets are super
7:36
Keys obviously this is also invalid
7:39
because when any subset is a Super Key
7:41
is not valid then obviously all subsets
7:43
are super cases also not valid let's
7:46
take the same example name and phone
7:47
number name separately and phone number
7:49
separately are subsets
7:51
and obviously the subset name and the
7:54
subset phone number are not super keys
7:56
at all and that's why the option b all
7:58
subsets are super case is not correct
8:01
and coming to option C no proper subset
8:04
is a Super Key which is true you know
8:07
why let's take this example name SSN and
8:10
phone number when we take this as a set
8:12
name SSN and phone number we can have
8:15
multiple subsets for this name alone
8:17
name and SSN name and phone number can
8:20
you see name and phone number so what I
8:22
mean to say here is when I take this as
8:25
the subset
8:26
and we can see there is a Super Key or
8:28
superset which has additional attribute
8:31
when we take name SSN and phone number
8:33
we have name phone number here we have
8:35
additional attribute here so the right
8:37
answer in simple terms the subset of a
8:40
set cannot be the same set and candidate
8:43
keys are set from Super Key which cannot
8:46
be the whole of the superset so option C
8:49
is the right answer here and option D is
8:51
obviously not correct because option C
8:53
is already the correct answer so the
8:56
right answer for the question the subset
8:58
of super key is a candidate key under
9:00
what condition and the condition is no
9:02
proper subset is a Super Key we are done
9:05
with question number five let's move on
9:07
to question number six the question is
9:09
we can Define more than one primary key
9:12
in a single table
9:14
and this is a true or false question and
9:17
the right answer for this question is
9:19
false because we cannot Define more than
9:22
one primary key in a single table we may
9:25
have multiple candidate keys but one of
9:27
the candidate Keys only will be chosen
9:29
as a primary key and hence we can Define
9:32
only one primary key in a single table
9:35
we are done with question number six
9:37
let's move on to the last question the
9:39
question is which key is used to make
9:42
relationship between two tables and the
9:45
options are option a Super Key option b
9:48
candidate key option C primary key and
9:51
option D foreign key and it's very easy
9:54
to answer this question because when we
9:56	talk about two tables and the
9:57	relationship among two tables obviously
10:00	option D foreign key is used
10:03	and that's it guys I hope you liked this
10:05	video and thank you for watching
10:08	[Applause]
10:10	[Music]
10:17	thank you
10:18	[Music]