Understanding Database Keys and Integrity

Rushdi Shams, Dept of CSE, KUET 1
Database SystemsDatabase Systems
Understanding KeysUnderstanding Keys
Entity IntegrityEntity Integrity
Referential IntegrityReferential Integrity
Propagation ConstraintPropagation Constraint
Version 1.0Version 1.0

2Rushdi Shams, Dept of CSE, KUET
What is keyWhat is key
 A key is a CONSTRAINT in field/ column in aA key is a CONSTRAINT in field/ column in a
tabletable
 Generally, keys are essential in database systemsGenerally, keys are essential in database systems
to invoke some integrityto invoke some integrity
 By a key, you can tag a field in a table so that theBy a key, you can tag a field in a table so that the
field can be used to relate tablesfield can be used to relate tables

Types of keyTypes of key
There are 3 types of keys in general. They are-There are 3 types of keys in general. They are-
 Primary keyPrimary key
 Unique keyUnique key
 Foreign keyForeign key

Primary keyPrimary key
 Used to uniquely identify a row in a tableUsed to uniquely identify a row in a table
 Also calledAlso called Surrogate keySurrogate key
 Say, you have a table of names of the students ofSay, you have a table of names of the students of
your class and their ages. How can you differyour class and their ages. How can you differ
between them? Because many of you have thebetween them? Because many of you have the
same name! don’t you? So, what happens if yousame name! don’t you? So, what happens if you
try to find out a name “Rezwana” and you findtry to find out a name “Rezwana” and you find
there are 5 Rezwanas in the class?there are 5 Rezwanas in the class?
 That invokes the idea of primary keyThat invokes the idea of primary key

Primary key (continued)Primary key (continued)
 This problem won’t be serious if you had anyThis problem won’t be serious if you had any
identifying criteria to differ between 5 Rezwanas.identifying criteria to differ between 5 Rezwanas.
 You still can identify 5 Rezwanas by adding anYou still can identify 5 Rezwanas by adding an
extra column in that table. Can you say whatextra column in that table. Can you say what
field that might be?field that might be?
 That is your roll number! So, 5 Rezwanas can beThat is your roll number! So, 5 Rezwanas can be
easily identifed by their roll numbers.easily identifed by their roll numbers.
 So, roll number is the primary key here.So, roll number is the primary key here.

Unique KeyUnique Key
 Unique keys are applied on the column of aUnique keys are applied on the column of a
tabletable
 Any field that is UNIQUE key, you will have toAny field that is UNIQUE key, you will have to
remember that no 2 entries in that column inremember that no 2 entries in that column in
that table will be the samethat table will be the same
 So, are you thinking “why there are primary keysSo, are you thinking “why there are primary keys
and unique keys? They are same, no?”and unique keys? They are same, no?”

Unique key (continued)Unique key (continued)
 No, they are not the same. In real life, when you areNo, they are not the same. In real life, when you are
creating tables, you sometimes will not want to let thecreating tables, you sometimes will not want to let the
table have the same entry twice (even though that is nottable have the same entry twice (even though that is not
the primary key)the primary key)
 An example can be, if you have a course table withAn example can be, if you have a course table with
primary key COURSE ID and another field COURSEprimary key COURSE ID and another field COURSE
NAME- take a look! You will not allow the data entryNAME- take a look! You will not allow the data entry
operator to entry the same subject name twice, do you?operator to entry the same subject name twice, do you?
 So, making COURSE NAME as UNIQUE will giveSo, making COURSE NAME as UNIQUE will give
you that opportunityyou that opportunity

Foreign KeyForeign Key
 Foreign keys are copies of primary key in theForeign keys are copies of primary key in the
child tablechild table
 When two tables are in relation, then child tableWhen two tables are in relation, then child table
is that table which depends on the other.is that table which depends on the other.

Foreign key (continued)Foreign key (continued)

Creating Primary keysCreating Primary keys
CREATE TABLE student(CREATE TABLE student(
student_idstudent_id number(10)number(10) NOT NULLNOT NULL,,
student_namestudent_name varchar(20),varchar(20),
PRIMARY KEY(student_id)PRIMARY KEY(student_id)
););

Creating Unique keysCreating Unique keys
CREATE TABLE course(CREATE TABLE course(
course_idcourse_id varchar(10)varchar(10) NOT NULL,NOT NULL,
course_namecourse_name varchar(20)varchar(20) UNIQUEUNIQUE,,
PRIMARY KEY(course_id)PRIMARY KEY(course_id)
););

Creating Foreign keysCreating Foreign keys
CREATE TABLE address(CREATE TABLE address(
student_idstudent_idnumber(10)number(10) NOT NULL,NOT NULL,
student_addressstudent_address varchar(30),varchar(30),
postcodepostcode varchar(10),varchar(10),
PRIMARY KEY (student_id),PRIMARY KEY (student_id),
FOREIGN KEY(student_id) REFERENCES studentFOREIGN KEY(student_id) REFERENCES student
););
BackBack

Case StudyCase Study

Case study (continued)Case study (continued)

Case study (continued)Case study (continued)
CREATE TABLE Band(CREATE TABLE Band(
band_idband_id varchar(10)varchar(10)
NOT NULL,NOT NULL,
band_nameband_name char(20),char(20),
PRIMARY KEY(band_id)PRIMARY KEY(band_id)
););
CREATE TABLE Track(CREATE TABLE Track(
track_idtrack_id varchar(10)varchar(10)
NOT NULL,NOT NULL,
band_idband_id varchar(10),varchar(10),
track_nametrack_name char(20),char(20),
descriptiondescription varchar(50),varchar(50),
PRIMARY KEY(track_id),PRIMARY KEY(track_id),
FOREIGN KEY (band_id)FOREIGN KEY (band_id)
REFERENCES BandREFERENCES Band
););

Important Fact!Important Fact!
 A child table may not have any primary key ofA child table may not have any primary key of
its own. It can make its foreign keys as itsits own. It can make its foreign keys as its
primary keys as well!primary keys as well!

Entity IntegrityEntity Integrity
 Entity integrity concerns with primary keyEntity integrity concerns with primary key
 It states that “Every table must have a primaryIt states that “Every table must have a primary
key” and “the column(s) that make up thekey” and “the column(s) that make up the
primary key must be unique to identify a rowprimary key must be unique to identify a row
and NOT NULL”and NOT NULL”

Referential Integrity & PropagationReferential Integrity & Propagation
ConstraintConstraint
 It ensures the integrity of referential relationshipsIt ensures the integrity of referential relationships
between tablesbetween tables
 In a relation between 2 tables, one table has primary keyIn a relation between 2 tables, one table has primary key
and the other a foreign keyand the other a foreign key
 Referential integrity ensures the integrity of the valuesReferential integrity ensures the integrity of the values
of primary and foreign key of parent and child tableof primary and foreign key of parent and child table
 Most database engines call these criteriaMost database engines call these criteria
CONSTRAINTS. Primary and foreign keys are alsoCONSTRAINTS. Primary and foreign keys are also
constraintsconstraints

Constraint (continued)Constraint (continued)
 A primary key table is assumed as parent tableA primary key table is assumed as parent table
and a foreign key table is assumed as child tableand a foreign key table is assumed as child table
 If a value in foreign key column in child table isIf a value in foreign key column in child table is
entered, it MUST be present in primary keyentered, it MUST be present in primary key
column in parent tablecolumn in parent table
 Foreign key can have NULL values, but primaryForeign key can have NULL values, but primary
key can NEVER have NULL valueskey can NEVER have NULL values

Constraint (continued)Constraint (continued)
 If the primary key value is changed it should beIf the primary key value is changed it should be
CASCADED to force to change foreign key values asCASCADED to force to change foreign key values as
wellwell
 Changing a value in foreign key must check firstly theChanging a value in foreign key must check firstly the
existence of its primary key. If the foreign key isexistence of its primary key. If the foreign key is
changed to NULL, no primary key is required. If it ischanged to NULL, no primary key is required. If it is
changed to NON NULL, the check for primary key ischanged to NON NULL, the check for primary key is
requiredrequired
 If deletion of primary key is required, it is required toIf deletion of primary key is required, it is required to
delete foreign key along with it (CASCADE) or foreigndelete foreign key along with it (CASCADE) or foreign
key firstly then primary keykey firstly then primary key

Referencial Integrity (continued)Referencial Integrity (continued)
 Creating referential integrity is easy! You haveCreating referential integrity is easy! You have
already done that. Look at the REFERENCESalready done that. Look at the REFERENCES
keyword here. That is ensuring your referentialkeyword here. That is ensuring your referential
integrity.integrity.
 Creating Foreign keysCreating Foreign keys

Propagation Constraint (continued)Propagation Constraint (continued)
 A propagation constraint is a rule about what to do if a row of a table that isA propagation constraint is a rule about what to do if a row of a table that is
referenced is modified or even deleted.referenced is modified or even deleted.
CREATE TABLE address(CREATE TABLE address(
student_idstudent_id number(10)number(10) NOT NULL,NOT NULL,
student_addressstudent_address varchar(30),varchar(30),
postcodepostcode varchar(10),varchar(10),
PRIMARY KEY (student_id),PRIMARY KEY (student_id),
FOREIGN KEY(student_id) REFERENCES studentFOREIGN KEY(student_id) REFERENCES student
ON DELETE CASCADEON DELETE CASCADE
););
This simply means that if parent table’s student_id is removed, all the childThis simply means that if parent table’s student_id is removed, all the child
table’s student_id s are removed!table’s student_id s are removed!

Propagation Action/ ReferentialPropagation Action/ Referential
ActionAction
 NO ACTIONNO ACTION:: This corresponds to the restricted
option. It rejects the delete action on the table
containing the primary key reference by the foreign key.
This is the default setting in the absence of a specified
ON DELETE constraint
 CASCADE: This corresponds to the cascades option.
It causes automatic deletion/ modification of associated
rows in the foreign key table following deletion/
modification of the row or rows in the primary key
table

Propagation Action/ ReferentialPropagation Action/ Referential
Action (continued)Action (continued)
 SET DEFAULT:SET DEFAULT: This causes deletion of the
row in the primary key table followed by setting
the foreign key values to the default specified on
the foreign key
 SET NULL:SET NULL: This corresponds to the nullifies
option. Following deletion of rows in the
primary key table it sets the associated foreign
key values to NULL.

Super keySuper key
 A super key is a set of columns within a tableA super key is a set of columns within a table
whose values can be used to identify a rowwhose values can be used to identify a row
uniquely.uniquely.
 In real world scenario, it is sometimes difficultIn real world scenario, it is sometimes difficult
to identify primary keys. Then super key is usedto identify primary keys. Then super key is used
to unique identification of rows.to unique identification of rows.

Super key (continued)Super key (continued)
 In this example, what if the roll is just like 1-60.In this example, what if the roll is just like 1-60.
you have this table for all 4 years’ students! Howyou have this table for all 4 years’ students! How
will you identify them by the roll?will you identify them by the roll?
 So, you can have combinations like {roll, name},So, you can have combinations like {roll, name},
{roll, name, dept}, {roll, name, year, semester},{roll, name, dept}, {roll, name, year, semester},
etc. to uniquely identify them! These areetc. to uniquely identify them! These are
superkeys.superkeys.
RollRoll NameName DeptDept YearYear SemesterSemester

Candidate keyCandidate key
 It is theIt is the minimal setminimal set of super key.of super key.
 Say, from the previous example, you only canSay, from the previous example, you only can
have super keys as followings (though you mayhave super keys as followings (though you may
have other combinations, but just shorten thathave other combinations, but just shorten that
this time)-this time)-
{roll, name}, {roll, name, dept}, {roll, name,{roll, name}, {roll, name, dept}, {roll, name,
year, semester}year, semester}
In this case, the candidate key is {roll, name}!In this case, the candidate key is {roll, name}!

Compound keyCompound key
 If you make a key with 2 or more columns, thenIf you make a key with 2 or more columns, then
that is called a compound keythat is called a compound key
 Also calledAlso called Composite keyComposite key oror concatenated keyconcatenated key
 Any one, none, or all, of the multiple attributesAny one, none, or all, of the multiple attributes
within the compound key can be foreign keyswithin the compound key can be foreign keys

Alternate keyAlternate key
 An alternate key is any of the candidate keys thatAn alternate key is any of the candidate keys that
was not chosen as primary keywas not chosen as primary key
 Say, you have 5 columns A, B, C, D, E and 3Say, you have 5 columns A, B, C, D, E and 3
candidate keys {A, B}, {A, C}, {A, D}, {B, C}candidate keys {A, B}, {A, C}, {A, D}, {B, C}
and {D, E}. You have chosen {A, B} as theand {D, E}. You have chosen {A, B} as the
primary key. So, {A, C}, {A, D}, {B, C} andprimary key. So, {A, C}, {A, D}, {B, C} and
{D, E} will be alternate keys{D, E} will be alternate keys
 Also known asAlso known as Secondary keySecondary key
 Strictly used in data retrieval purposeStrictly used in data retrieval purpose

Relational Database KeysRelational Database Keys

ReferencesReferences
 Database Systems:Database Systems:
Design, Implementation, and Management,Design, Implementation, and Management,
Sixth Edition, Rob and CoronelSixth Edition, Rob and Coronel
 www.wikipedia.orgwww.wikipedia.org
 Beginning Database Design by Gavin Powell,Beginning Database Design by Gavin Powell,
Wrox Publications, 2005Wrox Publications, 2005
 Database Systems by Paul Beynon-Devies,Database Systems by Paul Beynon-Devies,
Palgrave Macmillan, 2004Palgrave Macmillan, 2004

Understanding Database Keys and Integrity

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