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Database Management System 10CS54 VTU unit-3
UNIT 3 The Relational Data Model and Relational Database Constraints
3.1 Relational Model Concepts
Domain: A (usually named) set/universe of atomic values, where by "atomic" we mean
simply that, from the point of view of the database, each value in the domain is
indivisible (i.e., cannot be broken down into component parts).
Examples of domains (some taken from page 147):
o USA_phone_number: string of digits of length ten
o SSN: string of digits of length nine
o Name: string of characters beginning with an upper case letter
o GPA: a real number between 0.0 and 4.0
o Sex: a member of the set { female, male }
o Dept_Code: a member of the set { CMPS, MATH, ENGL, PHYS, PSYC, ... }
These are all logical descriptions of domains. For implementation purposes, it is
necessary to provide descriptions of domains in terms of concrete data types (or
formats) that are provided by the DBMS (such as String, int, boolean), in a manner
analogous to how programming languages have intrinsic data types.
Attribute: the name of the role played by some value (coming from some domain) in the
context of a relational schema. The domain of attribute A is denoted dom(A).
Tuple: A tuple is a mapping from attributes to values drawn from the respective domains
of those attributes. A tuple is intended to describe some entity (or relationship between
entities) in the miniworld.
As an example, a tuple for a PERSON entity might be
{ Name --> "Rumpelstiltskin", Sex --> Male, IQ --> 143 }
Relation: A (named) set of tuples all of the same form (i.e., having the same set of
attributes). The term table is a loose synonym. (Some database purists would argue that a
table is "only" a physical manifestation of a relation.)
Relational Schema: used for describing (the structure of) a relation. E.g., R(A1, A2, ...,
An) says that R is a relation with attributes A1, ... An. The degree of a relation is the
number of attributes it has, here n.
Example: STUDENT(Name, SSN, Address).
One would think that a "complete" relational schema would also specify the domain of
each attribute.
Relational Database: A collection of relations, each one consistent with its specified
relational schema.
3.1.2 Characteristics of Relations
Ordering of Tuples: A relation is a set of tuples; hence, there is no order associated with them.
That is, it makes no sense to refer to, for example, the 5th tuple in a relation. When a relation is
depicted as a table, the tuples are necessarily listed in some order, of course, but you should
attach no significance to that order. Similarly, when tuples are represented on a storage device,
they must be organized in some fashion, and it may be advantageous, from a performance
standpoint, to organize them in a way that depends upon their content.
Ordering of Attributes: A tuple is best viewed as a mapping from its attributes (i.e., the names
we give to the roles played by the values comprising the tuple) to the corresponding values.
Hence, the order in which the attributes are listed in a table is irrelevant. (Note that,
unfortunately, the set theoretic operations in relational algebra (at least how E&N define them)
make implicit use of the order of the attributes. Hence, E&N view attributes as being arranged as
a sequence rather than a set.)
Values of Attributes: For a relation to be in First Normal Form, each of its attribute domains
must consist of atomic (neither composite nor multi-valued) values. Much of the theory
underlying the relational model was based upon this assumption. Chapter 10 addresses the issue
of including non-atomic values in domains. (Note that in the latest edition of C.J. Date's book, he
explicitly argues against this idea, admitting that he has been mistaken in the past.)
The Null value: used for don't know, not applicable.
Interpretation of a Relation: Each relation can be viewed as a predicate and each tuple in that
relation can be viewed as an assertion for which that predicate is satisfied (i.e., has value true)
for the combination of values in it. In other words, each tuple represents a fact. Example (see
Figure 5.1): The first tuple listed means: There exists a student having name Benjamin Bayer,
having SSN 305-61-2435, having age 19, etc.
Keep in mind that some relations represent facts about entities (e.g., students) whereas others
represent facts about relationships (between entities). (e.g., students and course sections).
The closed world assumption states that the only true facts about the miniworld are those
represented by whatever tuples currently populate the database.
3.1.3 Relational Model Notation:
R(A1, A2, ..., An) is a relational schema of degree n denoting that there is a relation R
having as its attributes A1, A2, ..., An
By convention, Q, R, and S denote relation names.
By convention, q, r, and s denote relation states. For example, r(R) denotes one possible
state of relation R. If R is understood from context, this could be written, more simply, as
r.
By convention, t, u, and v denote tuples.
The "dot notation" R.A (e.g., STUDENT.Name) is used to qualify an attribute name,
usually for the purpose of distinguishing it from a same-named attribute in a different
relation (e.g., DEPARTMENT.Name).
3.2 Relational Model Constraints and Relational Database Schemas
Constraints on databases can be categorized as follows:
inherent model-based: Example: no two tuples in a relation can be duplicates (because a
relation is a set of tuples)
schema-based: can be expressed using DDL; this kind is the focus of this section.
application-based: are specific to the "business rules" of the miniworld and typically
difficult or impossible to express and enforce within the data model. Hence, it is left to
application programs to enforce.
Elaborating upon schema-based constraints:
3.2.1 Domain Constraints: Each attribute value must be either null (which is really a non-value)
or drawn from the domain of that attribute. Note that some DBMS's allow you to impose the not
null constraint upon an attribute, which is to say that that attribute may not have the (non-)value
null.
3.2.2 Key Constraints: A relation is a set of tuples, and each tuple's "identity" is given by the
values of its attributes. Hence, it makes no sense for two tuples in a relation to be identical
(because then the two tuples are actually one and the same tuple). That is, no two tuples may
have the same combination of values in their attributes.
Usually the miniworld dictates that there be (proper) subsets of attributes for which no two tuples
may have the same combination of values. Such a set of attributes is called a superkey of its
relation. From the fact that no two tuples can be identical, it follows that the set of all attributes
of a relation constitutes a superkey of that relation.
A key is a minimal superkey, i.e., a superkey such that, if we were to remove any of its
attributes, the resulting set of attributes fails to be a superkey.
Example: Suppose that we stipulate that a faculty member is uniquely identified by Name and
Address and also by Name and Department, but by no single one of the three attributes
mentioned. Then { Name, Address, Department } is a (non-minimal) superkey and each of {
Name, Address } and { Name, Department } is a key (i.e., minimal superkey).
Candidate key: any key! (Hence, it is not clear what distinguishes a key from a candidate key.)
Primary key: a key chosen to act as the means by which to identify tuples in a relation.
Typically, one prefers a primary key to be one having as few attributes as possible.
3.2.3 Relational Databases and Relational Database Schemas
A relational database schema is a set of schemas for its relations
together with a set of integrity constraints.
A relational database state/instance/snapshot is a set of states of its relations such that no
integrity constraint is violated.
3.2.4 Entity Integrity, Referential Integrity, and Foreign Keys
Entity Integrity Constraint: In a tuple, none of the values of the attributes forming the
relation's primary key may have the (non-)value null. Or is it that at least one such attribute must
have a non-null value? In my opinion, E&N do not make it clear!
Referential Integrity Constraint: (See Figure 5.7) A foreign key of relation R is a set of its
attributes intended to be used (by each tuple in R) for identifying/referring to a tuple in some
relation S. (R is called the referencing relation and S the referenced relation.) For this to make
sense, the set of attributes of R forming the foreign key should "correspond to" some superkey of
S. Indeed, by definition we require this superkey to be the primary key of S.
This constraint says that, for every tuple in R, the tuple in S to which it refers must actually be in
S. Note that a foreign key may refer to a tuple in the same relation and that a foreign key may be
part of a primary key (indeed, for weak entity types, this will always occur). A foreign key may
have value null (necessarily in all its attributes??), in which case it does not refer to any tuple in
the referenced relation.
Semantic Integrity Constraints: application-specific restrictions that are unlikely to be
expressible in DDL. Examples:
salary of a supervisee cannot be greater than that of her/his supervisor
salary of an employee cannot be lowered
3.3 Update Operations and Dealing with Constraint Violations
For each of the update operations (Insert, Delete, and Update), we consider what kinds of
constraint violations may result from applying it and how we might choose to react.
3.3.1 Insert:
domain constraint violation: some attribute value is not of correct domain
entity integrity violation: key of new tuple is null.
key constraint violation: key of new tuple is same as existing one
referential integrity violation: foreign key of new tuple refers to non-existent tuple
Ways of dealing with it: reject the attempt to insert! Or give user opportunity to try again with
different attribute values.
3.3.2 Delete:
Referential integrity violation: a tuple referring to the deleted one exists.
Three options for dealing with it:
Reject the deletion
Attempt to cascade (or propagate) by deleting any referencing tuples (plus those that
reference them, etc., etc.)
modify the foreign key attribute values in referencing tuples to null or to some valid
value referencing a different tuple
3.3.3 Update:
Key constraint violation: primary key is changed so as to become same as another tuple's
referential integrity violation:
o foreign key is changed and new one refers to nonexistent tuple
o primary key is changed and now other tuples that had referred to this one violate
the constraint
3.3.4 Transactions: This concept is relevant in the context where multiple users and/or
application programs are accessing and updating the database concurrently. A transaction is a
logical unit of work that may involve several accesses and/or updates to the database (such as
what might be required to reserve several seats on an airplane flight). The point is that, even
though several transactions might be processed concurrently, the end result must be as though
the transactions were carried out sequentially. (Example of simultaneous withdrawals from same
checking account.)
he Relational Algebra
Operations to manipulate relations.
Used to specify retrieval requests (queries).
Query result is in the form of a relation
3.4 Relational Operations:
SELECT and PROJECT operations.
Set operations: These include UNION U, INTERSECTION | |, DIFFERENCE -, CARTESIAN
PRODUCT X.
JOIN operations .
Other relational operations: DIVISION, OUTER JOIN, AGGREGATE FUNCTIONS.
3.4.1 SELECT and PROJECT
SELECT ):
Selects the tuples (rows) from a relation R that satisfy a certain selection condition c
Form of the operation:
c
The condition c is an arbitrary Boolean expression on the attributes of R
Resulting relation has the same attributes as R
Resulting relation includes each tuple in r(R) whose attribute values satisfy the condition
Examples:
DNO=4(EMPLOYEE)
SALARY>30000(EMPLOYEE)
(DNO=4 AND SALARY>25000) OR DNO=5(EMPLOYEE)
PROJECT operation (denoted by ):
Keeps only certain attributes (columns) from a relation R specified in an attribute list L
Form of operation: L
(R)
Resulting relation has only those attributes of R specified in L
The PROJECT operation eliminates duplicate tuples in the resulting relation so that it
remains a mathematical set (no duplicate elements).
Example: SEX,SALARY(EMPLOYEE)
If several male employees have salary 30000, only a single tuple <M, 30000> is kept in the
resulting relation
Duplicate tuples are eliminated by the operation.
Sequences of operations:
Several operations can be combined to form a relational algebra expression (query)
Example: Retrieve the names and salaries of employees who work in department 4:
FNAME,LNAME,SALARY ( DNO=4(EMPLOYEE) )
Alternatively, we specify explicit intermediate relations for each
step:
DEPT4_EMPS DNO=4(EMPLOYEE)
FNAME,LNAME,SALARY(DEPT4_EMPS)
Attributes can optionally be renamed in the resulting left-hand-side relation (this may be
required for some operations that will be presented later):
DEPT4_EMPS DNO=4(EMPLOYEE)
(FIRSTNAME,LASTNAME,SALARY) FNAME,LNAME,SALARY(DEPT4_EMPS)
3.5 Relational algebra operation Set theory Operations
Binary operations from mathematical set theory:
UNION: R1 R2,
INTERSECTION: R1 R2,
SET DIFFERENCE: R1 - R2,
CARTESIAN PRODUCT: R1 X R2.
For , , -, the operand relations R1(A1, A2, ..., An) and R2(B1, B2, ..., Bn) must have the same
number of attributes, and the domains of corresponding attributes must be compatible; that is,
dom(Ai) = dom(Bi) for i=1, 2, ..., n. This condition is called union compatibility. The resulting
relation for , , or - has the same attribute names as the first operand relation R1 (by
convention
CARTESIAN PRODUCT
R(A1
, A2
, ..., Am
, B1
, B2
, ..., Bn
) R1
(A1
, A2
, ..., Am
) X R2
(B1
, B2
, ..., Bn
)
A tuple t exists in R for each combination of tuples t1 from R1 and
t2 from R2 such that:
t[A1
, A2
, ..., Am
] = t1
and t[B1
, B2
, ..., Bn
] = t2
If R1 has n1 tuples and R2 has n2 tuples, then R will have n1*n2 tuples.
CARTESIAN PRODUCT is a meaningless operation on its own. It can combine related tuples
from two relations if followed by the appropriate SELECT operation.
Example: Combine each DEPARTMENT tuple with the EMPLOYEE tuple of the manager.
DEP_EMP DEPARTMENT X EMPLOYEE
DEPT_MANAGER MGRSSN=SSN(DEP_EMP)
3.6 JOIN Operations
THETA JOIN: Similar to a CARTESIAN PRODUCT followed by a SELECT. The condition c
is called a join condition.
R(A1
, A2
, ..., Am
, B1
, B2
, ..., Bn
) R1
(A1
, A2
, ..., Am
)
c
R2
(B1
, B2
, ..., Bn
)
EQUIJOIN: The join condition c includes one or more equality comparisons involving
attributes from R1 and R2. That is, c is of the form:
(Ai=Bj
) AND ... AND (Ah=Bk); 1<i,h<m, 1<j,k<n
In the above EQUIJOIN operation:
Ai
, ..., Ah
are called the join attributes of R1
Bj
, ..., Bk are called the join attributes of R2
Example of using EQUIJOIN:
Retrieve each DEPARTMENT's name and its manager's name:
T DEPARTMENT MGRSSN = SSN EMPLOYEE
RESULT DNAME,FNAME,LNAME (T)
NATURAL JOIN (*):
In an EQUIJOIN R R1 c R2
, the join attribute of R2 appear redundantly in the result relation
R. In a NATURAL JOIN, the redundant join attributes of R2 are eliminated from R. The
equality condition is implied and need not be specified.
R R1 *(join attributes of R1),(join attributes of R2) R2
Example: Retrieve each EMPLOYEE's name and the name of the DEPARTMENT he/she works
for:
T EMPLOYEE *(DNO),(DNUMBER) DEPARTMENT
RESULT FNAME,LNAME,DNAME (T)
If the join attributes have the same names in both relations, they need not be specified and we
can write R R1 * R2.
Example: Retrieve each EMPLOYEE's name and the name of his/her SUPERVISOR:
SUPERVISOR(SUPERSSN,SFN,SLN) SSN,FNAME,LNAME(EMPLOYEE)
T EMPLOYEE * SUPERVISOR
RESULT FNAME,LNAME,SFN,SLN(T)
Note: In the original definition of NATURAL JOIN, the join attributes were required to have
the same names in both relations.
There can be a more than one set of join attributes with a different meaning between the same
two relations. For example:
JOIN ATTRIBUTES
RELATIONSHIP
EMPLOYEE.SSN= EMPLOYEE manages
DEPARTMENT.MGRSSN
the DEPARTMENT
EMPLOYEE.DNO= EMPLOYEE works for
DEPARTMENT.DNUMBER the DEPARTMENT
Example: Retrieve each EMPLOYEE's name and the name of the DEPARTMENT he/she works
for:
T EMPLOYEE DNO=DNUMBER DEPARTMENT
RESULT FNAME,LNAME,DNAME(T)
A relation can have a set of join attributes to join it with itself :
JOIN ATTRIBUTES RELATIONSHIP
EMPLOYEE(1).SUPERSSN= EMPLOYEE(2) supervises
EMPLOYEE(2).SSN EMPLOYEE(1)
One can think of this as joining two distinct copies of the relation, although only one relation
actually exists In this case, renaming can be useful.
Example: Retrieve each EMPLOYEE's name and the name of his/her SUPERVISOR:
SUPERVISOR(SSSN,SFN,SLN) SSN,FNAME,LNAME(EMPLOYEE)
T EMPLOYEE SUPERSSN=SSSNSUPERVISOR
RESULT FNAME,LNAME,SFN,SLN(T)
Complete Set of Relational Algebra Operations:
All the operations discussed so far can be described as a sequence of only the operations
SELECT, PROJECT, UNION, SET DIFFERENCE, and CARTESIAN PRODUCT.
Hence, the set { , , , - , X } is called a complete set of relational algebra operations. Any
query language equivalent to these operations is called relationally complete.
For database applications, additional operations are needed that were not part of the original
relational algebra. These include:
1. Aggregate functions and grouping.
2. OUTER JOIN and OUTER UNION.
3.7 Additional Relational Operations
AGGREGATE FUNCTIONS ( )
Functions such as SUM, COUNT, AVERAGE, MIN, MAX are often applied to sets of values or
sets of tuples in database applications
<grouping attributes> <function list>(R)
The grouping attributes are optional
Example 1: Retrieve the average salary of all employees (no grouping needed):
(AVGSAL) AVERAGE SALARY (EMPLOYEE)
Example 2: For each department, retrieve the department number, the number of employees, and
the average salary (in the department):
(DNO,NUMEMPS,AVGSAL) DNO COUNT SSN, AVERAGE SALARY (EMPLOYEE)
DNO is called the grouping attribute in the above example
OUTER JOIN
In a regular EQUIJOIN or NATURAL JOIN operation, tuples in R1 or R2 that do not have
matching tuples in the other relation do not appear in the result
Some queries require all tuples in R1 (or R2 or both) to appear in
the result
When no matching tuples are found, nulls are placed for the
missing attributes
LEFT OUTER JOIN: R1 X R2 lets every tuple in R1 appear in the result
RIGHT OUTER JOIN: R1 X R2 lets every tuple in R2
appear in the result
FULL OUTER JOIN: R1 X R2 lets every tuple in R1 or R2 appear in the result
3.8 Examples of Queries in Relational Algebra
Q1: Retrieve the name and address of all employees who work for the ‘Research’
department.
RESEARCH_DEPT DNAME=’Research’ (DEPARTMENT)
RESEARCH_EMPS (RESEARCH_DEPT DNUMBER=
DNOEMPLOYEEEMPLOYEE)
RESULT FNAME, LNAME, ADDRESS (RESEARCH_EMPS)
Q6: Retrieve the names of employees who have no dependents.
ALL_EMPS SSN(EMPLOYEE)
EMPS_WITH_DEPS(SSN) ESSN(DEPENDENT)
EMPS_WITHOUT_DEPS (ALL_EMPS - EMPS_WITH_DEPS)
RESULT LNAME, FNAME (EMPS_WITHOUT_DEPS * EMPLOYEE)
3.9 Relational Database Design Using ER-to-Relational Mapping
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