Transcript The Relational Model

The Relational Model
Chapter 3
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Why Study the Relational Model?
• Most widely used model.
• Vendors: IBM, Informix, Microsoft, Oracle,
Sybase, etc.
• “Legacy systems” in older models
• E.G., IBM’s IMS
• Recent competitor: object-oriented model
• ObjectStore, Versant, Ontos
• A synthesis emerging: object-relational model
• Informix Universal Server, UniSQL, O2, Oracle, DB2
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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The SQL Query Language
• Developed by IBM (system R) in the 1970s
• Need for a standard since it is used by many
vendors
• Standards:
•
•
•
•
SQL-86
SQL-89 (minor revision)
SQL-92 (major revision, current standard)
SQL-99 (major extensions)
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Summary Relational Data Model I
1.
2.
3.
A relation database consists of a set of relations (tables)
Each relation consist of a set of attribute/domain pairs
Attributes have to be single-valued. Additionally, “null”
values are supported
4. The attributes of a relation are unordered
5. Attribute values have to be atomic (cannot be tuples or
relations)
6. Relations store sets of tuples (queries return bags of tuples!!);
each tuple of a relation stores a value for each attribute of the
relation; all tuples stored in a relation must be different
7. Tuples in a relation are unordered
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Summary Relational Data Model II
8.
Each relation has one primary key that consists of a set of
attributes that uniquely identifies the tuples in a relation).
9. Relationships between object in the real world are represented
in the relational data model by “exporting primary keys of the
objects that participate in the relationship”. The exported keys
are called foreign keys.
10. Multi-values attributes can be presented either by using a
separate relation or by representing the object through
multiple tuples (one for each value of the multi-valued
attribute).
11. Optional attributes (attributes that not necessarily have a
value) can be represented using null values; furthermore, they
can also be represented by using a separate relation that stores
the relation’s primary key attributes and the optional
attribute.
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Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
Creating Relations in SQL
• Creates the Students
CREATE TABLE Students
(sid: CHAR(20),
relation. Observe that the
name: CHAR(20),
type (domain) of each field
login: CHAR(10),
is specified, and enforced by
age: INTEGER,
the DBMS whenever tuples
gpa: REAL)
are added or modified.
• As another example, the
CREATE TABLE Enrolled
Enrolled table holds
(sid: CHAR(20),
information about courses
cid: CHAR(20),
that students take.
grade: CHAR(2))
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Primary Key Constraints
• A set of fields is a candidate key for a relation if :
1. No two distinct tuples can have same values in all key fields,
and
2. This is not true for any subset of the candidate key.
• Part 2 false? A superkey.
• If there’s >1 key for a relation, one of the keys is chosen (by
DBA) to be the primary key.
• E.g., sid is a candidate key for Students. (What about
name?) The set {sid, gpa} is a superkey.
• Remark: Sometimes people say ‘key’ when the refer
to candidate keys.
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Primary and Candidate Keys in SQL
• Possibly many candidate keys (specified using
UNIQUE), one of which is chosen as the primary key.
v
v
“For a given student and course, CREATE TABLE Enrolled
(sid CHAR(20)
there is a single grade.” vs.
cid CHAR(20),
“Students can take only one
grade CHAR(2),
course, and receive a single grade
PRIMARY KEY (sid,cid) )
for that course; further, no two
CREATE TABLE Enrolled
students in a course receive the
(sid CHAR(20)
same grade.”
cid CHAR(20),
Used carelessly, an IC can prevent
grade CHAR(2),
the storage of database instances
PRIMARY KEY (sid),
that arise in practice!
UNIQUE (cid, grade) )
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Foreign Keys, Referential Integrity
• Foreign key : Set of fields in one relation that is used
to `refer’ to a tuple in another relation. (Must
correspond to primary key of the second relation.)
Like a `logical pointer’.
• E.g. sid is a foreign key referring to Students:
• Enrolled(sid: string, cid: string, grade: string)
• If all foreign key constraints are enforced, referential
integrity is achieved, i.e., no dangling references.
• Can you name a data model w/o referential integrity?
• Links in HTML!
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Foreign Keys in SQL
• Only students listed in the Students relation should
be allowed to enroll for courses.
CREATE TABLE Enrolled
(sid CHAR(20), cid CHAR(20), grade CHAR(2),
PRIMARY KEY (sid,cid),
FOREIGN KEY (sid) REFERENCES Students )
Enrolled
sid
53666
53666
53650
53666
cid
grade
Carnatic101
C
Reggae203
B
Topology112
A
History105
B
Students
sid
53666
53688
53650
name
login
Jones jones@cs
Smith smith@eecs
Smith smith@math
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
age
18
18
19
gpa
3.4
3.2
3.8
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Enforcing Referential Integrity
• Consider Students and Enrolled; sid in Enrolled is a
foreign key that references Students.
• What should be done if an Enrolled tuple with a nonexistent student id is inserted? (Reject it!)
• What should be done if a Students tuple is deleted?
•
•
•
•
Also delete all Enrolled tuples that refer to it.
Disallow deletion of a Students tuple that is referred to.
Set sid in Enrolled tuples that refer to it to a default sid.
(In SQL, also: Set sid in Enrolled tuples that refer to it to a
special value null, denoting `unknown’ or `inapplicable’.)
• Similar if primary key of Students tuple is updated.
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Referential Integrity in SQL/92
• SQL/92 supports all 4
CREATE TABLE Enrolled
options on deletes and
(sid CHAR(20),
updates.
cid CHAR(20),
grade CHAR(2),
• Default is NO ACTION
PRIMARY KEY (sid,cid),
(delete/update is rejected)
FOREIGN KEY (sid)
• CASCADE (also delete
REFERENCES Students
all tuples that refer to
ON DELETE CASCADE
deleted tuple)
ON UPDATE SET DEFAULT )
• SET NULL / SET DEFAULT
(sets foreign key value
of referencing tuple)
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Graphical Short Notations for
Relational Schemas
• R(A,B,C), S(D,E) meaning: {A,B} is a primary key for
R; D is a primary key for S
• S(D,E) meaning: X is a foreign key in T that references
attribute D of relation T: T[X]  S[D]
T(X,Y,Z)
Remark: The graphical short notation only specifies relation
names, attributes, primary keys, and foreign keys but omits
other schema information (such as attribute domains, uniqueness
constraints, …)
Relational Data Model, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Relational Integrity Rules
• Entity Integrity: Null values are disallowed for primary key attributes
of a relation.
• Referential Integrity: the value of a foreign key must be either null or
there must be a tuple with the foreign key value for the exported
reference attribute(s) in the referenced relation.
• Stability of the Primary Key: Updates of primary key attributes
should be disallowed.
• Remark: The referenced attribute of a foreign key is usually the
primary key of the relation; however, SQL-92 allows foreign keys to
reference any attribute(s) of a relation, even attributes that are not
superkeys. Syntax:
FOREIGN KEY <attribute-list> REFERENCES
<relation><attribute-list>
e.g. FOREIGN KEY (hssn,wssn,from) REFERENCES
Wedding(husband,wife,from)
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