The Relational Model Constraints and SQL DDL

Week 2-3

Weeks 2-3 MIE253-Consens 1 Schedule Week Date Lecture Topic This week’s 1 Jan 9 Introduction to Data Management reading: 2 Jan 16 The Relational Model Chapter 3 3 Jan. 23 Constraints and SQL DDL 4 Jan. 30 DB Applications, JDBC 5 Feb 6 Relational Algebra Last week’s 6 Feb 13 Advanced SQL reading: - Feb 20 [Reading Week] Chapters 1-2 7 Feb 27 Review and Midterm (Mar 1) 8Mar5OLAP 9 Mar 12 ER Conceptual Modelling 10 Mar 19 Normalization 11 Mar 26 XML and Data Integration 12 Apr 2 Transactions and the Internet, Query Processing 13 Apr 9 Final Review

Weeks 2-3 MIE253-Consens 2 Relational Database Instance

 Finite set of relations  Each relation consists of a schema and an instance  Database schema  set of relation schemas  constraints among relations (inter-relational constraints)  Database instance  set of (corresponding) relation instances

Weeks 2-3 MIE253-Consens 3 Types of DBs in an Organization

Weeks 2-3 MIE253-Consens 4 DB Budgets

Platform Average Cost

Excel $ 500 Access Individual $ 3,000 Access Simple Multi-user $ 10,000 Access Workgroup/Department $ 50,000 VB/VS.NET/Java and SQL Server $ 500,000 Oracle, IBM DB2 $ 2,000,000 SAP, etc. $ 10,000,000

Weeks 2-3 MIE253-Consens 5 Number of DBs in Organizations

Platform Quantity Excel 50,000 Access Individual 5,000 Access Simple Multi-user 1,000 Access Department 500 VB/VS.NET/Java and SQL Server 50 Oracle, IBM DB2 25 SAP, etc. 10

Weeks 2-3 MIE253-Consens 6 SQL DDL Data Definition Language (DDL)  Sublanguage of SQL for describing schema, manipulates the definition of tables stored in the DBMS catalog  Current version of the ANSI/ISO standard is SQL:2008 (supports Objects, XML)  SQL-92 is the most commonly used subset  Database vendors generally deviate from standard, but eventually converge  e.g., SQL in Access, SQL in JDBC

Weeks 2-3 MIE253-Consens 7 Table Declaration

CREATE TABLE Student (  Incorporates the Id INTEGER, table definition Name CHAR(20), Address CHAR(50), to the database Status CHAR(10) schema )

Student Id Name Address Status 101222333 John 10 Cedar St Freshman 234567890 Mary 22 Main St Sophomore

Weeks 2-3 MIE253-Consens 8 Integrity Constraints

 Part of schema  Restriction on state (or of sequence of states) of data base  Enforced by DBMS  Protects database from errors  Enforces enterprise rules  Intra-relational - involve only one relation  Part of relation schema  e.g., all Ids are unique  Inter-relational - involve several relations  Part of relation schema or database schema

Weeks 2-3 MIE253-Consens 9 Kinds of Integrity Constraints

 Static – restricts legal states of database  Syntactic (structural)  e.g., all values in a column must be unique  Semantic (involve meaning of attributes)  e.g., cannot register for more than 18 credits  Dynamic – limitation on sequences of database states  e.g., cannot raise salary by more than 5%

Weeks 2-3 MIE253-Consens 10 Key Constraint

 A key constraint is a sequence of attributes A1,…,An (n=1 possible) of a relation schema, S, with the following property:  A relation instance s of S satisfies the key constraint iff at most one row in s can contain a particular set of values, a1,…,an, for the attributes A1,…,An  Minimality: no subset of A1,…,An is a key constraint  Key  Set of attributes mentioned in a key constraint  e.g., Id in Student,  e.g., (StudId, CrsCode, Semester) in Transcript  It is minimal: no subset of a key is a key  (Id, Name) is not a key of Student

Weeks 2-3 MIE253-Consens 11 Key Constraint

Weeks 2-3 MIE253-Consens 12 Key Constraint (cont’d)

 Superkey - set of attributes containing key  (Id, Name) is a superkey of Student  Every relation has a key  Not with SQL bags  Relation can have several keys:  primary key: in Student (can’t be null)  candidate key: (Name, Address) in Student Id

Weeks 2-3 MIE253-Consens 13 Primary/Candidate Keys

CREATE TABLE Course ( CrsCode CHAR(6), CrsName CHAR(20), DeptId CHAR(4), Descr CHAR(100), PRIMARY KEY (CrsCode), UNIQUE (DeptId, CrsName) -- candidate key ) Things that start with 2 dashes are comments in the SQL standard -- not in Access

Weeks 2-3 MIE253-Consens 14 Example Database Schema

Student (Id: INT, Name: STRING, Address: STRING, Status: STRING)

Professor (Id: INT, Name: STRING, DeptId: DEPTS)

Department(DeptId: DEPTS, Name: STRING)

Course (DeptId: DEPTS, CrsName: STRING, CrsCode: COURSES)

Transcript (CrsCode: COURSES, StudId: INT, Grade: GRADES, Semester: SEMESTERS)

Weeks 2-3 MIE253-Consens 15 Foreign Key Constraint

 Referential integrity: Item named in one relation must refer to tuples that describe that item in another  Transcript (CrsCode) references Course(CrsCode )  Professor(DeptId) references Department(DeptId)

 Attribute A1 is a foreign key of R1 referring to attribute A2 in R2, if whenever there is a value v of A1, there is a tuple of R2 in which A2 has value v, and A2 is a key of R2

Weeks 2-3 MIE253-Consens 16 Foreign Key Constraint (Example)

Department Professor DeptId DeptId v3 v1 v5 v2 v1 v3 v6 v4 v2 -- v7 v3 v4

Foreign key Candidate key

Weeks 2-3 MIE253-Consens 17 Foreign Key (cont’d)

 Names of A1 and A2 need not be the same.  With tables: Teaching(CrsCode: COURSES, Sem: SEMESTERS, ProfId: INT) Professor(Id: INT, Name: STRING, DeptId: DEPTS) ProfId attribute of Teaching references Id attribute of Professor  R1 and R2 need not be distinct.  Employee(Id:INT, MgrId:INT, ….)  Employee(MgrId) references Employee(Id)  Every manager is also an employee and hence has a unique row in Employee

Weeks 2-3 MIE253-Consens 18 Foreign Key (cont’d)

 Foreign key might consist of several columns  (CrsCode, Semester) of Transcript references (CrsCode, Semester) of Teaching

 R1(A1, …An) references R2(B1, …Bn)  There exists a 1 - 1 correspondance between

A1,…An and B1,…Bn

 Ai and Bi have same domains (although not necessarily the same names)

 B1,…,Bn is a candidate key of R2

Weeks 2-3 MIE253-Consens 19 Foreign Key Constraint

CREATE TABLE Teaching ( ProfId INTEGER, CrsCode CHAR (6), Semester CHAR (6), PRIMARY KEY (CrsCode, Semester), FOREIGN KEY (CrsCode) REFERENCES Course, FOREIGN KEY (ProfId) REFERENCES Professor (Id) )

Weeks 2-3 MIE253-Consens 20 Foreign Key Constraint

CrsCode

CrsCode ProfId c

c p Course Id Teaching

p Professor

Weeks 2-3 MIE253-Consens 21 Constraints and SQL DDL

Week 2-3 (cont.)

Weeks 2-3 MIE253-Consens 22 Schedule Week Date Lecture Topic This week’s 1 Jan 9 Introduction to Data Management reading: 2 Jan 16 The Relational Model Chapter 5 (5.3) 3 Jan. 23 Constraints and SQL DDL Chapter 8 (8.5, 4 Jan. 30 SQL DML, DB Applications, JDBC 8.2.5) 5 Feb 6 Relational Algebra 6 Feb 13 Advanced SQL - Feb 20 [Reading Week] 7 Feb 27 Review and Midterm (Mar 1) 8Mar5OLAP Previous 9 Mar 12 ER Conceptual Modelling week’s: 10 Mar 19 Normalization Chapters 1-2 11 Mar 26 XML and Data Integration Chapter 3 12 Apr 2 Transactions and the Internet, Query Processing 13 Apr 9 Final Review

Weeks 2-3 MIE253-Consens 23 NULL

 Problem: Not all information might be known when row is inserted (e.g., Grade might be missing from Transcript)  A column might not be applicable for a particular row (e.g., MaidenName if row describes a male)  Solution: Use place holder – NULL  Not a value of any domain (although called null value)  Indicates the absence of a value  Not allowed in certain situations  Primary keys and columns constrained by NOT NULL

Weeks 2-3 MIE253-Consens 24 Default Value

-Value to be assigned if attribute value in a row is not specified

CREATE TABLE Student ( Id: INTEGER, Name CHAR(20) NOT NULL, Address CHAR(50), Status CHAR(10) DEFAULT ‘freshman’, PRIMARY KEY (Id) )

Weeks 2-3 MIE253-Consens 25 Semantic Constraints in SQL

 Primary key and foreign key are examples of structural (syntactic) constraints  Semantic constraints  Express the logic of the application at hand:  e.g., number of registered students  maximum enrollment

Weeks 2-3 MIE253-Consens 26 Semantic Constraints (cont’d)

 Used for application dependent conditions  Example: limit attribute values

CREATE TABLE Transcript ( StudId INTEGER, CrsCode CHAR(6), Semester CHAR(6), Grade CHAR(1), CHECK (Grade IN (‘A’, ‘B’, ‘C’, ‘D’, ‘F’)), CHECK (StudId > 0 AND StudId < 1000000000) )

 Each row in table must satisfy condition

Weeks 2-3 MIE253-Consens 27 Domains

 Possible attribute values can be specified  Using a CHECK constraint or  Creating a new domain  Domain can be used in several declarations  Domain is a schema element

CREATE DOMAIN Grades CHAR (1) CHECK (VALUE IN (‘A’, ‘B’, ‘C’, ‘D’, ‘F’)) CREATE TABLE Transcript ( …., Grade Grades, … )

Weeks 2-3 MIE253-Consens 28 Modifying the Schema

ALTER TABLE Student ADD COLUMN Gpa INTEGER DEFAULT 0

ALTER TABLE Student ADD CONSTRAINT GpaRange CHECK (Gpa >= 0 AND Gpa <= 4)

ALTER TABLE Student DROP CONSTRAINT GpaRange-- constraint names are useful

DROP TABLE Employee

Weeks 2-3 MIE253-Consens 29 Circularity in Foreign Key Constraint

A1 A2 A3 B1 B2 B3

A y x xyB

candidate key: A1 candidate key: B1 foreign key: A3 references B(B1) foreign key: B3 references A(A1)

Problem 1: Creation of A requires existence of B and vice versa Solution: CREATE TABLE A ( ……) -- no foreign key CREATE TABLE B ( ……) -- include foreign key ALTER TABLE A ADD CONSTRAINT cons

FOREIGN KEY (A3) REFERENCES B (B1)

MIE253-Samavi 30 Inclusion Dependency

 Referential integrity constraint that is not a foreign key constraint  Teaching(CrsCode, Semester) references Transcript(CrsCode, Semester) (no courses are taught without students)  Target attributes do not form a candidate key in Transcript (StudId missing)  No simple enforcement mechanism for inclusion dependencies in SQL (requires assertions)

Weeks 2-3 MIE253-Consens 31 Summary: The Relational Model

 Schemas: physical, conceptual, external  Data model: DSL, DDL, DML  Relational Model, SQL  Relation Instance (tuples, cardinality)  Relation Schema (relation name, attributes, domains, constraints)  Constraints (Keys, PK, CK, FK, ID)  SQL DDL

Weeks 2-3 MIE253-Consens 32