EECE 433 Chapter 3, Spring 2012

Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke The Relational Model Chapter 3 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Outline  What is a relation and how to crate it using SQL  How to access/retrieve/modify data from DBMS using SQL  SQL commands to destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Relational Database: Definitions  Relational database: a set of relations  Relation: made up of 2 parts:  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).  Instance of the schema: a table, with rows and columns. #Rows = cardinality, #fields = degree / arity.  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 Example Instance of Students Relation sid name login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@eecs 18 3.2 53650 Smith smith@math 19 3.8  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 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Outline  What is a relation and how to crate it using SQL  How to access/retrieve/modify data from DBMS using SQL  SQL commands to create/destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke SQL Overview (1/2)  SQL = Structured Query Language  SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition Language (DDL).  The query and update commands form the DML part of SQL:  SELECT - extracts data from a database  UPDATE - updates data in a database  DELETE - deletes data from a database  INSERT INTO - inserts new data into a database Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 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 sid name login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@ee 18 3.2 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke Querying Multiple Relations  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” S.name E.cid Smith Topology112 sid cid grade 53831 Carnatic101 C 53831 Reggae203 B 53650 Topology112 A 53666 History105 B Given the following instance of Enrolled : we get: Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 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! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Modifying column values (ie rows) UPDATE Students S SET S.age = S.age + 1, S.gpa = S.gpa – 1 WHERE S.sid = 53688  Note: The WHERE is applied first to find the desired row, then SET is applied to change the specific fields. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Try SQL http://www.w3schools.com/SQL/sql_t ryit.asp Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Outline  What is a relation and how to crate it using SQL  How to access/retrieve data from DBMS using SQL  SQL commands to create/destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 SQL Overview (2/2)  The DDL part of SQL permits database tables to be created or deleted. The most important DDL statements in SQL are:  CREATE DATABASE - creates a new database  ALTER DATABASE - modifies a database  CREATE TABLE - creates a new table  ALTER TABLE - modifies a table  DROP TABLE - deletes a table  CREATE INDEX - creates an index (search key)  DROP INDEX - deletes an index Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 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.  As another example, the Enrolled table holds information about courses that students take. CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL) CREATE TABLE Enrolled (sid: CHAR(20), cid: CHAR(20), grade: CHAR(2)) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 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. You could also DROP COLUMN. ALTER TABLE Students ADD COLUMN firstYear INTEGER Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Outline  What is a relation and how to crate it using SQL  How to access/retrieve data from DBMS using SQL  SQL commands to destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 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 1 Primary Key Constraints  A set of fields is a 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 key. (ie no subset fields of the key can be a key)  If there’s >1 key for a relation, one of the keys is chosen (by DBA) to be the primary key.  A key combined with another field is called a superkey.  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 2 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) ) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 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! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 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 ) sid name login age gpa 53666 Jones jones@cs 18 3.4 53688 Smith smith@eecs 18 3.2 53650 Smith smith@math 19 3.8 sid cid grade 53666 Carnatic101 C 53666 Reggae203 B 53650 Topology112 A 53666 History105 B Enrolled Students Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Outline  What is a relation and how to crate it using SQL  How to access/retrieve data from DBMS using SQL  SQL commands to destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 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 non- existent 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 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 (sets foreign key value of referencing tuple) CREATE TABLE Enrolled (sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ON DELETE CASCADE ON UPDATE SET DEFAULT ) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Outline  What is a relation and how to crate it using SQL  How to access/retrieve data from DBMS using SQL  SQL commands to destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Logical DB Design: ER to Relational  Entity sets to tables - straightforward CREATE TABLE Employees (ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn)) Employees ssn name lot Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Relationship Sets to Tables  In translating a relationship set to a relation, attributes of the relation must include:  All descriptive attributes.  Keys for each participating entity set (as foreign keys). • This set of attributes forms a superkey for the relation. 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) dname budget did since lot name ssn Works_In Employees Departments • Simple case: No participation constraint Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Translating Many to one constraint  Each dept has at most one manager, according to the key constraint on Manages. Translation to relational model? dname budget did since lot name ssn Manages Employees Departments We can not have two rows with same did Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 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) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 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 budget did since name dname budget did since Manages since Departments Employees ssn Works_In Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Participation Constraints in SQL  We can capture participation constraints involving one entity set in a binary relationship, but little else (without resorting to CHECK constraints). CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE NO ACTION) Participation Many to 1 Can not delete employee if pointed by dept Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Works_in particpation?  How would you implement it using CHECK(..) constraint. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Can we have total participation with the first relation model? CREATE TABLE Manages( ssn CHAR(11) NOT NULL, did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments) Key learning: For many to one relationship, combine entity with relationship into one relation. Especially useful when we have total participation constraint. Does this work? Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Review: Weak Entities  A weak entity can be identified uniquely only by considering the primary key of another (owner) entity.  Owner entity set and weak entity set must participate in a one-to-many relationship set (1 owner, many weak entities).  Weak entity set must have total participation in this identifying relationship set. lot name age pname Dependents Employees ssn Policy cost Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Translating Weak Entity Sets  Weak entity set and identifying relationship set are translated into a single table.  When the owner entity is deleted, all owned weak entities must also be deleted. CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE) Many to 1 Weak entity Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 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 3 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Review: Binary vs. Ternary Relationships  Each policy is owned by just 1 employee, and each dependent is tied to the covering policy.  Every policy has an owner. age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 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? Add ssn to policyid 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 4 Outline  What is a relation and how to crate it using SQL  How to access/retrieve data from DBMS using SQL  SQL commands to destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 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<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. Use: RESTRICT to disallow or CASCADE to drop the views as well. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 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. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Outline  What is a relation and how to crate it using SQL  How to access/retrieve data from DBMS using SQL  SQL commands to destroy/change tables  Integrity Constraints  How DBMS enforces integrity constraints  Converting ER diagram to Relation  Views using queries  Internet shop example Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Case Study (cont-d): Conceptual Schema address cid isbn price Yr_publish cname Customers Books Orders Order_date Ship_Date qty cardnum Qty_in_stock Author title Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Internet Shop Example  CREATE TABLE Books( isbn CHAR(10), title CHAR(80), author CHAR(80), qty_in_stock INTEGRER, price REAL, year_published INTEGER, PRIMARY KEY(isbn))  CREATE TABLE Orders( isbn CHAR(10), cid INTEGER, cardnum CHAR(16), qty INTEGER, order_date DATE, ship_date DATE, PRIMARY KEY(isbn, cid), FOREIGN KEY (isbn) REFERENCES Books, FOREIGN KEY (cid) REFERENCES Customers) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Internet Shop Example  CREATE TABLE Customers(cid INTEGER, cname CHAR(80), address CHAR(200), PRIMARY KEY(cid)) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4 Internet Shop Example  New Requirement:  Purchase several books in a single order.  Customers should be able to distinguish between several orders placed in the same day. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5 Internet Shop Example  Add an ordernum .  The combination of ordernum and ISBN will uinquely identify an order. So we can have a combination of multiple ISBNS for the same order  Note that the ordernum also uniquely identifies cid.  CREATE TABLE Orders( ordernum INTEGER, isbn CHAR(10), cid INTEGER, cardnum CHAR(16), qty INTEGER, order_date DATE, ship_date DATE, PRIMARY KEY(isbn, ordernum), FOREIGN KEY (isbn) REFERENCES Books, FOREIGN KEY (cid) REFERENCES Customers) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5 Internet Shop Example  New Requirement:  Protect credit card info from customer access  CREATE VIEW OrderInfo(isbn, cid, qty, order_date, ship_date) AS SELECT O.isbn, O.cid, O.qty, O.order_date, O.ship_date FROM ORDERS O Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5 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