COP4710 Database Systems Relational Model

ER Model vs. Relational Model Both are used to model data ER model has many concepts Entities, relationships, attributes, etc. Well-suited for capturing the app. requirements Not well-suited for computer implementation Relational model Has just a single concept: relation World is represented with a collection of tables Well-suited for efficient manipulations on computers

Behind the Scene: It’s All About Modeling 1973 Charles W. Bachman 1981 Edgar F. Codd For his fundamental and continuing contributions to the theory and practice of database management systems 1998 James Gray 2014 Michael Stonebraker We definitely need more

Name of Table (Relation) Column (Field, Attribute) Relation: An Example Name of Table (Relation) Column (Field, Attribute) Products Name Price Category Manufacturer Gizmo 19.99 Gadgets Gizmo works Power gizmo 29.99 Single touch 149.99 Photography Canon Multi touch 203.99 househould Hitachi Row (Record, Tuple) Domain (Atomic type)

Relations Schema vs. instance = columns vs. rows Schema of a relation: R(A1, A2, …, Ak) Relation name Attribute names Attribute types (domains) Schema of a database: R1(…), R2(…),…, Rn(…) A set of relation schemas Questions When do you determine a schema (instance)? How often do you change your mind?

Relations The database maintains a current database state Updates to the data happen very frequently Insert a tuple delete a tuple update an attribute in a tuple Updates to the schema are relatively rare, and rather painful. Why?

Defining a Database Schema A database schema comprises declarations for the relations (“tables”) of the database Simplest form of creation is: CREATE TABLE <name> ( <list of elements> ); And you may remove a relation from the database schema by: DROP TABLE <name>;

Elements of Table Declarations The principal element is a pair consisting of an attribute and a type The most common types are: INT or INTEGER (synonyms) REAL or FLOAT (synonyms) CHAR(n ) = fixed-length string of n characters VARCHAR(n ) = variable-length string of up to n characters

Example: Create Table CREATE TABLE Sells ( bar CHAR(20), beer VARCHAR(20), price REAL );

Dates and Times DATE and TIME are types in SQL The form of a date value is DATE ‘yyyy-mm-dd’ Example: DATE ‘2002-09-30’ for Sept. 30, 2002 The form of a time value is TIME ‘hh:mm:ss’ with an optional decimal point and fractions of a second following Example: TIME ‘15:30:02.5’ = two and a half seconds after 3:30PM

Declaring Keys An attribute or list of attributes may be declared PRIMARY KEY or UNIQUE Each says the attribute(s) so declared functionally determines all the attributes of the relation schema Single attribute keys CREATE TABLE Beers ( name CHAR(20) UNIQUE, manf CHAR(20) );

Multi-attribute Keys CREATE TABLE Sells ( bar CHAR(20), beer VARCHAR(20), price REAL, PRIMARY KEY (bar, beer) );

PRIMARY KEY vs. UNIQUE Standard SQL requires these distinctions There can be only one PRIMARY KEY for a relation, but several UNIQUE attributes No attribute of a PRIMARY KEY can ever be NULL in any tuple. But attributes declared UNIQUE may have NULL’s, and there may be several tuples with NULL SQL standard also allows DBMS implementers to make their own distinctions between PRIMARY KEY and UNIQUE Example: some DBMS might automatically create an index (data structure to speed search) in response to PRIMARY KEY, but not UNIQUE

Other Declarations for Attributes Two other declarations we can make for an attribute are: NOT NULL means that the value for this attribute may never be NULL DEFAULT <value> says that if there is no specific value known for this attribute’s component in some tuple, use the stated <value> CREATE TABLE Drinkers ( name CHAR(30) PRIMARY KEY, addr CHAR(50) DEFAULT ‘123 Monroe St.’, phone CHAR(16) );

Example for NULL and DEFAULT Suppose we insert the fact that Sally is a drinker, but we know neither her address nor her phone An INSERT with a partial list of attributes makes the insertion possible: INSERT INTO Drinkers(name) VALUES(‘Sally’); If we had declared phone NOT NULL, this insertion would have been rejected Name Address Phone ‘Sally’ ‘123 Monroe St.’ NULL

Foreign Keys A Foreign Key is a field whose values are keys in another relation Must correspond to primary key of the second relation Like a `logical pointer’ Enrolled Students CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students, FOREIGN KEY (cid) REFERENCES Courses )

Referential Integrity Consider relations 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 to NULL Similar if primary key of Students tuple is updated

Adding Attributes We may change a relation schema by adding a new attribute (“column”) by: ALTER TABLE <name> ADD <attribute declaration>; Example: ALTER TABLE Bars ADD phone CHAR(16) DEFAULT ‘unlisted’;

Deleting Attributes Remove an attribute from a relation schema by: ALTER TABLE <name> DROP <attribute>; Example: we don’t really need the license attribute for bars: ALTER TABLE Bars DROP license;

Translating ER Diagram to Rel. Design Basic cases entity set E = relation with attributes of E relationship R = relation with attributes being keys of related entity sets + attributes of R Special cases combining two relations translating weak entity sets translating is-a relationships and subclasses

Entity Set to Relation Employees ssn name lot CREATE TABLE Employees ( ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY(ssn) )

Relationship Set to Relation In translating a many-to-many 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 )

Translating ER Diagrams since lot name ssn dname did budget Manages Employees Departments Translation to relational model? 1-to-1 1-to-Many Many-to-1 Many-to-Many

Translating ER Diagrams Map relationship set to a (virtual) Manages 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 )

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)

Another Example loginname hostname IP Logins At Hosts time Hosts(hostname, IP) Logins(loginname, hostname, time) At(loginName, hostName) At becomes part of Logins

Translating ISA Hierarchies to Relations name ssn lot Employees hourly_wages hours_worked ISA contractid Hourly_Emps Contract_Emps 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)

Translating ISA Hierarchies to Relations General approach (ER 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 (OO approach) Hourly_Emps: ssn, name, lot, hourly_wages, hours_worked Each employee must be in one of these two subclasses

Summary: ER Diagram to Relations Relational schema e.g.account=(bname, acct_no, bal) E/R diagram E E = ( a1, …, an ) a1 ….. an E1 R1 E2 R1= ( a1, b1, c1, …, ck ) a1 …. an c1 …. ck b1 …. bm

Summary: ER Diagram to Relations a1 …. an c1 …. ck b1 …. bm Could have : R1= ( a1, b1, c1, …, ck ) Put b1 as the key for R1, it is also the key for E2=(b1, …., bn) Usual strategy (combination) ignore R1 Add a1, c1, …., ck to E2 instead, i.e. E2=(b1, …., bn, a1, c1, …, ck)

Summary: ER Diagram to Relations ? R1 ? E2 E1 a1 …. an c1 …. ck b1 …. bm E1 = ( a1, …, an ) E2 = ( b1, …, bm ) R1 R1 = ( a1, b1, c1 …, ck ) E1 = ( a1, …, an ) R1 E2 = ( b1, …, bm , a1, c1, …, ck) E1 = ( a1, …, an , b1, c1, …, ck) R1 E2 = ( b1, …, bm ) R1 Treat as n:1 or 1:m

Summary: ER Diagram to Relations Weak Entity sets E1 = ( a1, …, an ) IR E1 E2 E2 = (a1, b1, …, bm ) a1 …. an b1 …. bm Sub-classes a1 … an Method 1 (ER): E = ( a1, …, an ) E1 S1 = (a1, b1, …, bm ) Isa S2 = ( a1, c1 …, ck ) Method 2 (OO): S1 S2 S1 = (a1,…, an, b1, …, bm ) c1 …. ck S2 = ( a1, …, an, c1 …, ck ) b1 …. bm