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Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1
The Relational Model
Chapter 3
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2
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, DB
Relational Database: Definitions
Relational database: a set of relations
Relation: made up of 2 parts:
Instance : a table , with rows and columns.
Rows = cardinality , # fields = degree / arity.
Schema : specifies name of relation, plus name and type of each column.
- E.G. Students( sid : string, name : string, login : string, age : integer, gpa : real).
Can think of a relation as a set of rows or
tuples (i.e., all rows are distinct).
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4
Example Instance of Students Relation
sid name login age gpa
53666 Jones jones@cs 18 3.
53688 Smith smith@eecs 18 3.
53650 Smith smith@math 19 3.
Cardinality = 3, degree = 5, all rows distinct
Do all columns in a relation instance have to
be distinct?
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5
Relational Query Languages
A major strength of the relational model:
supports simple, powerful querying of data.
Queries can be written intuitively, and the
DBMS is responsible for efficient evaluation.
The key: precise semantics for relational queries. Allows the optimizer to extensively re-order operations, and still ensure that the answer does not change.
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-
SQL-89 (minor revision) SQL-92 (major revision) SQL-99 (major extensions, current standard)
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 10
Destroying and Altering Relations
Destroys the relation Students. The schema
information and the tuples are deleted.
DROP TABLE Students
The schema of Students is altered by adding a
new field; every tuple in the current instance
is extended with a null value in the new field.
ALTER TABLE Students ADD COLUMN firstYear: integer
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 11
Adding and Deleting Tuples
Can insert a single tuple using:
INSERT INTO Students (sid, name, login, age, gpa) VALUES (53688, ‘Smith’, ‘smith@ee’, 18, 3.2)
Can delete all tuples satisfying some
condition (e.g., name = Smith):
DELETE FROM Students S WHERE S.name = ‘Smith’
- Powerful variants of these commands are available; more later!
Integrity Constraints (ICs)
IC: condition that must be true for any instance
of the database; e.g., domain constraints.
ICs are specified when schema is defined. ICs are checked when relations are modified.
A legal instance of a relation is one that satisfies
all specified ICs.
DBMS should not allow illegal instances.
If the DBMS checks ICs, stored data is more
faithful to real-world meaning.
Avoids data entry errors, too!
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 13
Primary Key Constraints
A set of fields is a key for a relation if :
- No two distinct tuples can have same values in all key fields, and
- This is not true for any subset of the 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 key for Students. (What about
name ?) The set { sid, gpa } is a superkey.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 14
Primary and Candidate Keys in SQL
Possibly many candidate keys (specified using
UNIQUE), one of which is chosen as the primary key.
CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid) )
“For a given student and course, there is a single grade.” vs. “Students can take only one course, and receive a single grade for that course; further, no two students in a course receive the same grade.” Used carelessly, an IC can prevent the storage of database instances that arise in practice!
CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) )
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?
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 19
Where do ICs Come From?
ICs are based upon the semantics of the real-
world enterprise that is being described in the
database relations.
We can check a database instance to see if an
IC is violated, but we can NEVER infer that
an IC is true by looking at an instance.
An IC is a statement about all possible instances! From example, we know name is not a key, but the assertion that sid is a key is given to us.
Key and foreign key ICs are the most
common; more general ICs supported too.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 20
Logical DB Design: ER to Relational
Entity sets to tables:
CREATE TABLE Employees (ssn CHAR(11), name CHAR(20), lot INTEGER, Employees^ PRIMARY KEY^ (ssn))
ssn
name lot
Relationship Sets to Tables
In translating a relationship set to a relation, attributes of the relation must include: Keys for each participating entity set (as foreign keys).
- This set of attributes forms a superkey for the relation. All descriptive attributes.
CREATE TABLE Works_In( ssn CHAR(1), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 22
Review: Key Constraints
Each dept has at most one manager, according to the key constraint on Manages.
Translation to relational model?
1-to-1 1-to Many Many-to-1 Many-to-Many
dname did budget
since
lot
name ssn
Employees Manages Departments
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 23
Translating ER Diagrams with Key Constraints
Map relationship to a table: Note that did is the key now! Separate tables for Employees and Departments. Since each department has a unique manager, we could instead combine Manages and Departments.
CREATE TABLE Manages( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)
CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees)
Review: Participation Constraints
Does every department have a manager?
If so, this is a participation constraint : the participation of Departments in Manages is said to be total (vs. partial ).
- Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!)
lot
name dname did budget
since name dname did budget
since
Manages
since
Employees Departments
ssn
Works_In
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 28
Review: ISA Hierarchies
Contract_Emps
name ssn
Employees
lot
hourly_wages ISA
Hourly_Emps
contractid
hours_worked As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity. Overlap constraints : Can Joe be an Hourly_Emps as well as a Contract_Emps entity? ( Allowed/disallowed ) Covering constraints : Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no)
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 29
Translating ISA Hierarchies to Relations
General approach:
3 relations: Employees, Hourly_Emps and Contract_Emps.
- Hourly_Emps : Every employee is recorded in Employees. For hourly emps, extra info recorded in Hourly_Emps ( hourly_wages , hours_worked , ssn) ; must delete Hourly_Emps tuple if referenced Employees tuple is deleted).
- Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes.
Alternative: Just Hourly_Emps and Contract_Emps.
Hourly_Emps : ssn , name, lot, hourly_wages, hours_worked. Each employee must be in one of these two subclasses_._
Review: Binary vs. Ternary
Relationships
What are the
additional
constraints in
the 2nd
diagram?
pname age
Covers Dependents
name
Employees
ssn (^) lot
Policies policyid (^) cost
Beneficiary
pname age
Dependents
policyid (^) cost
Policies
Purchaser
name
Employees
ssn lot
Bad design
Better design
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 31
Binary vs. Ternary Relationships (Contd.)
The key constraints allow us to combine Purchaser with Policies and Beneficiary with Dependents. Participation constraints lead to NOT NULL constraints. What if Policies is a weak entity set?
CREATE TABLE Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE) CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE)
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 32
Views
A view is just a relation, but we store a
definition , rather than a set of tuples.
CREATE VIEW YoungActiveStudents (name, grade) AS SELECT S.name, E.grade FROM Students S, Enrolled E WHERE S.sid = E.sid and S.age<
Views can be dropped using the DROP VIEW command.
How to handle DROP TABLE if there’s a view on the table?
- DROP TABLE command has options to let the user specify this.
Views and Security
Views can be used to present necessary
information (or a summary), while hiding
details in underlying relation(s).
Given YoungStudents, but not Students or Enrolled, we can find students s who have are enrolled, but not the cid’s of the courses they are enrolled in.
