COP 3540 - Introduction to Database Structures Relational Model

Relational Data Model The relational data model was first introduced in 1970 by E. F. Codd, then of IBM (Codd, 1970). 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 The relational data model represents data in the form of relations (or tables). The relational data model consists of the following three components: Data structure: Data are organized in the form of tables, with rows and columns. Data manipulation: Powerful operations (typically implemented using the SQL language) are used to manipulate data stored in the relations. Data integrity: The model includes mechanisms to specify business rules that maintain the integrity of data.

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 (or 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).

Relational Data Structure A relation is a named, two-dimensional table of data. Each relation (or table) consists of a set of named columns and an arbitrary number of unnamed rows.

Example Instance of Students Relation Cardinality = 3, degree = 5, all rows distinct. Do all columns in a relation instance have to be distinct?

Terminology Relation: A relation is a table with columns and rows. Attribute: An attribute is a named column of a relation. Domain: A domain is the set of allowable values for one or more attributes. Tuple: A tuple is a row of a relation. Degree: The degree of a relation is the number of attributes it contains. Cardinality: The cardinality of a relation is the number of tuples it contains.

Terminology Relation Schema : A named relation defined by a set of attribute and domain name pairs. Because every attribute has an associated domain, there are constraints (called domain constraints) that form restrictions on the set of values allowed for the attributes of relations. Relation Database Schema : A set of relation schemas, each with a distinct name.

Quiz

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) SQL-99 (major extensions) ….. SQL-2011

Structured Query Language (SQL) A database language should allow a user to: create the database and relation structures; perform basic data management tasks, such as the insertion, modification, and deletion of data from the relations; perform both simple and complex queries. Data Definition Language (DDL) for defining the database structure and controlling access to the data. Data Manipulation Language (DML) for retrieving and updating data.

Data Definition Language (DDL)

Creating Relations in SQL Creates the Students relation. Observe that the type (domain) of each field is specified, and enforced by the DBMS whenever tuples are added or modified. CREATE TABLE Students ( sid CHAR(20), name CHAR(20), login CHAR(10), age INTEGER, gpa REAL) As another example, the Enrolled table holds information about courses that students take. CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2))

Create Table

Destroying and Altering Relations DROP TABLE Students Destroys the relation Students. The schema information and the tuples are deleted. ALTER TABLE Students ADD COLUMN firstYear: integer 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.

Constraints The relational data model includes several types of constraints, or rules limiting acceptable values and actions, whose purpose is to facilitate maintaining the accuracy and integrity of data in the database. The major types of constraints are: Domain Constraints Entity Integrity Referential Integrity

Domain Constraints All of the values that appear in a column of a relation must be from the same domain. A domain is the set of values that may be assigned to an attribute. A domain definition usually consists of the following components: domain name, meaning, data type, size (or length), and allowable values or allowable range (if applicable).

Entity Integrity The entity integrity rule is designed to ensure that every relation has a primary key and that the data values for that primary key are all valid. In particular, it guarantees that every primary key attribute is non-null.

Referential Integrity A referential integrity constraint is a rule that maintains consistency among the rows of two relations. In the relational data model, associations between tables are defined through the use of foreign keys.

Relational Keys Superkey: An attribute, or set of attributes, that uniquely identifies a tuple within a relation. Candidate key: A superkey such that no proper subset is a superkey within the relation. A candidate key K for a relation R has two properties: Uniqueness. In each tuple of R, the values of K uniquely identify that tuple. Irreducibility. No proper subset of K has the uniqueness property. Primary key: The candidate key that is selected to identify tuples uniquely within the relation. Foreign key: An attribute, or set of attributes, within one relation that matches the candidate key of some (possibly the same) relation.

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 Example Students (sid, name, gpa) sid is a key. (What about name?) {sid, gpa} is a superkey.

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) ) CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) )

Key Constraints in SQL

Foreign Keys, Referential Integrity Foreign key : Set of columns 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’. CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid, cid), FOREIGN KEY (sid) REFERENCES Students (sid) )

Foreign Key Constraints

Data Manipulation Language (DML)

Insert Tuple

Delete Tuple

Update Tuple

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’

Insert Tuple

Delete Tuple

Update Tuple 19 2.2

Update Tuple 3.2 3.6

Enforcing Integrity Constraints Assume that sid is primary key

Enforcing Integrity Constraints Assume that sid is primary key

Enforcing Integrity Constraints Assume that sid is primary key

Enforcing Referential Constraints

Enforcing Referential Integrity

Referential Integrity in SQL SQL/92 and SQL:1999 support all 4 options on deletes and updates. Default is NO ACTION (delete/update is rejected) CASCADE (also delete all tuples that refer to deleted tuple) SET NULL SET DEFAULT CREATE TABLE Employee ( EmpId CHAR(20), EmpName CHAR(20), DeptId CHAR(20), PRIMARY KEY (EmpName), FOREIGN KEY (DeptId) REFERENCES Department (DeptId) ON DELETE CASCADE ON UPDATE SET DEFAULT )

Review Two relations and a referential integrity constraints

REFERENTIAL INTEGRITY CONSTRAINT DELETE RESTRICT and UPDATE RESTRICT CREATE TABLE employee( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department);

DELETE RESTRICT example

REFERENTIAL INTEGRITY CONSTRAINT DELETE CASCADE CREATE TABLE employee ( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department ON DELETE CASCADE);

DELETE CASCADE example

REFERENTIAL INTEGRITY CONSTRAINT DELETE CASCADE CREATE TABLE employee ( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department ON DELETE SET NULL);

DELETE SET-TO-NULL example

DELETE SET-TO-DEFAULT example

REFERENTIAL INTEGRITY CONSTRAINT DELETE RESTRICT and UPDATE RESTRICT CREATE TABLE employee ( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department);

UPDATE RESTRICT example

REFERENTIAL INTEGRITY CONSTRAINT UPDATE CASCADE CREATE TABLE employee ( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department ON UPDATE CASCADE);

UPDATE CASCADE example

REFERENTIAL INTEGRITY CONSTRAINT UPDATE SET-TO-NULL CREATE TABLE employee ( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department ON UPDATE SET NULL);

UPDATE SET-TO-NULL example

UPDATE SET-TO-DEFAULT example

REFERENTIAL INTEGRITY CONSTRAINT DELETE CASCADE and UPDATE SET-TO-NULL CREATE TABLE employee ( empid CHAR(4), empname CHAR(20), deptid CHAR(2), PRIMARY KEY (empid), FOREIGN KEY (deptid) REFERENCES department ON DELETE CASCADE ON UPDATE SET NULL);

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.

Logical DB Design: Entity to Relation (or Table) Use E-R model to get a high-level graphical view of essential components of enterprise and how they are related. Then convert E-R diagram to SQL DDL, or whatever database model you are using

Logical DB Design: Entity to Relation (or Table) Entity sets to tables: CREATE TABLE Employees ( ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn))

Logical DB Design: Entity to Relation (or Table) CREATE TABLE Departments ( )

Logical DB Design: Relationship to Table In translating a relationship to a relation, attributes of the relation must include: Keys for each participating entity set (as foreign keys). All descriptive attributes

Logical DB Design: Relationship to Table CREATE TABLE Works_In ( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)

Logical DB Design: Relationship to Table CREATE TABLE Manages ( )

Logical DB Design: Relationship to Table

Logical DB Design: Relationship Sets to Tables CREATE TABLE Works_In2 ( ssn CHAR(11), did INTEGER, address CHAR(20), since DATE, PRIMARY KEY (ssn, did, address), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments, FOREIGN KEY (address) REFERENCES Locations )

Logical DB Design: Relationship to Table

Logical DB Design: Relationship Sets to Tables

Review: Constraints Each department 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

Translating Relationship Sets with Key Constraints

Translating Relationship with Key Constraints CREATE TABLE Manages ( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees (ssn), FOREIGN KEY (did) REFERENCES Departments (did)) At most one ! Key Constraint

Translating Relationship with Key Constraints CREATE TABLE Dept_Mgr ( did INTEGER, dname CHAR(20), budget REAL, mgr_ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (mgr_ssn) REFERENCES Employees (ssn)) At most one ! Key Constraint

Review: Total Participation Constraint Every department should have a manager (i.e., exactly one manager). At most one ! Key Constraint At least one ! Total Participation

Review: Participation Constraints CREATE TABLE Dept_Mgr ( did INTEGER, dname CHAR(20), budget REAL, mgr_ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees ON DELETE NO ACTION ) At most one ! Key Constraint At least one ! Total Participation

Review: Weak Entities A weak entity can be identified uniquely only by considering the primary key of another (owner) entity.

Translating Weak Entity Sets CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, employee_ssn CHAR(11) NOT NULL, PRIMARY KEY (pname, employee_ssn), FOREIGN KEY (employee_ssn) REFERENCES Employees (ssn) ON DELETE CASCADE)

Review: Binary vs. Ternary Relationships age pname Dependents Covers name Employees ssn lot Policies policyid cost Bad design Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Better design What are the additional constraints in the 2nd diagram?

Review: Binary vs. Ternary Relationships Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot CREATE TABLE Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid), FOREIGN KEY (ssn) REFERENCES Employees ON DELETE CASCADE)

Binary vs. Ternary Relationships (Contd.) Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies ON DELETE CASCADE)

Data Manipulation Language (DML) Data Retrieval

The SQL Query Language To find all 18 year old students, we can write: SELECT * FROM Students S WHERE S.age=18 To find just names and logins, replace the first line: SELECT S.name, S.login FROM Students S WHERE S.age=18

Querying Multiple Relations (Join) What does the following query compute? SELECT S.name, E.cid FROM Students S, Enrolled E WHERE S.sid = E.sid AND E.grade=“A” Given the following instances of Enrolled and Students: We get:

Review

Review

Review

View 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 < 21 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.

View

Relational Model: Summary A tabular representation of data. Simple and intuitive, currently the most widely used. Integrity constraints can be specified by the DBA, based on application semantics. DBMS checks for violations. Two important ICs: primary and foreign keys In addition, we always have domain constraints. Powerful and natural query languages exist. Rules to translate ER to relational model