Database Relational Model The Relational Data Modeling ER

Database Relational Model

The Relational Data Modeling E/R diagrams Relational Schema Tables: column names: attributes rows: tuples Physical storage Complex file organization and index structures.

Relational Data Model n Core of majority of modern databases n Virtually all business relies on some form of relational database n Solid theoretical/mathematical foundation

Relational Model Concepts n The relational Model of Data is based on the concept of a Relation. n A Relation is a mathematical concept based on the ideas of sets. n Order of tuples not important n Order of attributes not important (in theory)

RELATION n RELATION is A table of values n A relation may be thought of as a set of rows. n A relation may alternately be though of as a set of columns.

Relation Instance Name Address Telephone Ahmed 123 Main St 555 -1234 Hassan 12 State St 555 -1235 Ahmed 123 Main St 555 -1235 Mona 456 Main St 555 -2221 Sally 456 Main St 555 -2223 Hassan 12 State St 555 -1235

Example State

Schema Student Course Grade Hermione Grainger Potions A- Draco Malfoy Potions B Harry Potter Potions A Ron Weasley Potions C n The schema of a relation is the name of the relation followed by a parenthesized list of attributes (+ types of attributes). Courses. Taken(Student, Course, Grade) n A design in a relational model consists of a set of schemas. n Such a set of schemas is called a relational database schema.

Example Schema

Relation Schema relation name set of attributes Stock. Item Attribute Item. ID Description Price Taxable attribute names Domain string(4) string(50) currency/dollars boolean attribute domains

Relational schema n Is a direct map from ER diagram into basic table n For example, the schema (ID, phone, name, birth_date, address)

Banking Example branch (branch_name, branch_city, assets) customer (customer_name, customer_street, customer_city) account (account_number, branch_name, balance) loan (loan_number, branch_name, amount) depositor (customer_name, account_number) borrower (customer_name, loan_number)

Example Database Schema n Student (Id: INT, Name: STRING, Address: STRING, Status: STRING) n Professor (Id: INT, Name: STRING, Dept. Id: DEPTS) n Course (Dept. Id: DEPTS, Crs. Name: STRING, Crs. Code: COURSES) n Transcript (Crs. Code: COURSES, Stud. Id: INT, Grade: GRADES, Semester: SEMESTERS) n Department (Dept. Id: DEPTS, Name: STRING) 13

Key Constraints n key of R: A set of attributes SK of R such that no two tuples will have the same value for SK n If a relation has several candidate keys, one is chosen arbitrarily to be the primary key. The primary key attributes are underlined. n Indicate a key by underlining the key attributes. n Example: If name is a key for Beers: Beers(name, manf)

Entity Set to Relation name category price Product(name, category, price) name gizmo category gadgets price $19. 99

Relationships to Relations price name category Start Year makes name Company Product Stock price Makes(product-name, product-category, company-name, year) Product-name gizmo Product-Category Company-name gadgets gizmo. Works Starting-year 1963

Relationships to Relations price name category Start Year makes name Company Product Stock price No need for Makes. Modify Product: name category price gizmo gadgets 19. 99 Start. Year company. Name 1963 gizmo. Works

ERD FROM DATASTORES FLIGHTS carries Flight Aircraft Flight (flight#, arrival_airport, depart_airport, arrival_time, depart_time) uses Airport (code, city) Aircraft (aircraft, no_of_seats) Identifier of flight seems strange. ‘Flight_no’ alone should identify a flight. Airport

ERD FROM DATASTORES FLIGHTS Aircraft Flight (flight#, arrival_airport, depart_airport, arrival_time, depart_time) Stopover (flight_no, code, arrival_time, depart_time) Airport (code, city) carries Aircraft (aircraft, no_of_seats) Stopover Flight Departs_from Stops_at Leaves_from Arrives_at Airport

Multi-way Relationships to Relations Product name address name price Purchase Store Person Purchase( ssn name , , )

name addr Drinkers 1 name Likes manf Beers 2 Buddies husband Favorite wife Married Likes(drinker, beer) Favorite(drinker, beer) Married(husband, wife) Buddies(name 1, name 2) n For one-one relation Married, we can choose either husband or wife as key.

Weak Entity Sets, Relationships Relations n Relation for a weak E. S. must include its full key (i. e. , attributes of related entity sets) as well as its own attributes. n A supporting (double-diamond) relationship yields a relation that is actually redundant and should be deleted from the database schema.

Representing Entity Sets With Simple Attributes n A strong entity set reduces to a schema with the same attributes student(ID, name, tot_cred) n A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set section ( course_id, sec_id, sem, year )

Example name Logins name @ Hosts(host. Name) Logins(login. Name, host. Name) At(login. Name, host. Name 2) n In At, host. Name and host. Name 2 must be the same host, so delete one of them. n Then, Logins and At become the same relation; delete one of them. n In this case, Hosts’ schema is a subset of Logins’ schema. Delete Hosts?

Converting Non-identifying Attributes n Single-valued (standard attribute) n Create a table column for each n Derived n Omit: these values are not stored in our tables n Later, we can produce these values using a query n Multi-valued n n Relational model does not directly support! However, as we have discussed, a multi-valued attribute can be conceptualized as a new (weak) entity, thus implying a separate table.

Composite and Multivalued Attributes n Composite attributes are flattened out by creating a separate attribute for each component attribute n Example: given entity set instructor with composite attribute name with component attributes first_name and last_name the schema corresponding to the entity set has two attributes name_first_name and name_last_name n Prefix omitted if there is no ambiguity n Ignoring multivalued attributes, extended instructor schema is n instructor(ID, first_name, middle_initial, last_name, street_number, street_name, apt_number, city, state, zip_code,

Composite and Multivalued Attributes n A multivalued attribute M of an entity E is represented by a separate schema EM n Schema EM has attributes corresponding to the primary key of E and an attribute corresponding to multivalued attribute M n Example: Multivalued attribute phone_number of instructor is represented by a schema: inst_phone= ( ID, phone_number) n Each value of the multivalued attribute maps to a separate tuple of the relation on schema EM n For example, an instructor entity with primary key 22222 and phone numbers 456 -7890 and 123 -4567 maps to two tuples: (22222, 456 -7890) and (22222, 123 -4567)

Converting Binary Relationships n One-to-one relationships n n Consider the 2 associated entity tables. The foreign key column(s) can be with either entity n As before, copy the primary key column(s) of the related table n Note: in a 1: 1 relationship, the two entities often use the same identifier, in which case the existing primary key columns serve the “dual role” of both primary and foreign keys n A separate foreign key column is then unnecessary!

Converting Binary Relationships n One-to-many relationships n n n Consider the 2 associated entity tables. Within the “many side” entity’s table, we need to have foreign key column(s) referring to the related “one side” entity instances We use the identifier of the related entity to define the foreign key column(s) n In other words, we include a column (a “copy”) of primary key values from the related table § The copied primary key values we call “foreign keys. ”

Schema Diagram

REVIEW Reverse engineer this relational schema to find an equivalent ER schema.

Converting Binary Relationships n Many-to-many relationships n n n Relational model does not directly support! However, each many-to-many relationship can be conceptualized as a new (associative) entity, thus implying a separate table. The identifier for the associative entity is the combination of the identifiers for the two related entities. n Thus, for the separate table we create for an M: M relationship, its primary key columns include the primary key columns for both of the related tables.

Finding the Keys If the relation comes from a many-many relationship, the key of the relation is the set of all attribute keys in the relations corresponding to the entity sets name Product Person buys price name date buys(name, ssn, date) ssn

PREVIEW: ER to Relational

EER Bank Schema

Step 1: Regular Entities n Regular entity types become relations n include all simple attributes n include only components of compound attributes n keys become primary keys n if multiple keys (candidates) select a primary key CUSTOMER(Ssn, Name, Addr, Phone)

Step 1: Regular Entities BANK(Code, Name, Addr) ACCOUNT(Acct_no, Type, Balance) LOAN(Loan_no, Type, Amount)

Step 2: Weak Entities n Weak entity types become relations n include all simple attributes n include only components of compound attributes n create a primary key from partial key and key of owning entity type (through identifying relationship) n attributes acquired through identifying relationship become a foreign key* * typically, deletions and insertions will be propagated through this foreign key

Step 2: Weak Entities n Weak entity types become relations BANK_BRANCH(Bank_code, Branch_No, Addr) FK BANK(Code, Name, Addr)

Step 3: Binary 1: 1 Relationships n Approach 1: Foreign Key EMPLOYEE(Ssn, Name, …) FK DEPARTMENT(Name, Number, Mgr_start_date)

Step 3: Binary 1: 1 Relationships n Approach 2: Merged Relation AJB(x, y, p, q, r) or AJB(x, y, p, q, r)

Step 4: Binary 1: N Relationships n 1: N Relationships become foreign key at N side n any relationship attributes also go to N side LOAN(Loan_no, Type, Amount, Bank, Branch) BANK_BRANCH(Bank_code, Branch_No, Addr)

Step 4: Binary 1: N Relationships n 1: N Relationships become foreign key at N side n any relationship attributes also go to N side ACCOUNT(Acct_no, Type, Balance, Bank, Branch) BANK_BRANCH(Bank_code, Branch_No, Addr)

Step 5: Binary M: N Relationships n M: N Relationships must become a new relation n contains FKs to both related entities n combined FKs become PK for new relations n relationship attributes go in new relation CUSTOMER(Ssn, Name, Addr, Phone) A_C(Acct, Cust) ACCOUNT(Acct_no, Type, Balance, Bank, Branch)

Step 6: Multivalued Attributes n Multivalued attributes must become new relations FK to associated entity type n PK is whole relation n DEPARTMENT(Name, Number, Mgr_start_date) DEPT_LOCATIONS(DName, Dno, Location)

Step 7: N-ary Relationships n Non-Binary Relationships become new relations n FKs to all participating entity types n Combine FKs to make a PK (exclude entities with max participation of 1) n Include any relationship attributes SUPPLIER(SName) PROJECT(Proj_name) PART(Part_no) SUPPLY(SName, Part, Quantity)

Completed Bank Schema CUSTOMER(Ssn, Name, Addr, Phone) BANK(Code, Name, Addr) ACCOUNT(Acct_no, Type, Balance, Bank, Branch) LOAN(Loan_no, Type, Amount, Bank, Branch) BANK_BRANCH(Bank_code, Branch_No, Addr) A_C(Acct, Cust) L_C(Loan, Cust) BANK_BRANCH(Bank_code) refers to BANK LOAN(Bank, Branch) refers to BANK_BRANCH ACCOUNT(Bank, Branch) refers to BANK_BRANCH A_C(Acct) refers to ACCOUNT A_C(Cust) refers to CUSTOMER L_C(Loan) refers to LOAN L_C(Cust) refers to CUSTOMER

Bank Schema: MS Access

Exercise A university database contains information about professors (identified by social security number) and courses (identified by courseid). Professors teach courses; each of the following situations concerns the Teaches relationship set. For each situation, draw an ER diagram that describes it. n Professors can teach the same course in several semesters, and each offering must be recorded. 49

Exercise Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded. 50

Exercise n Every professor teaches exactly one course (no more, no less), and every course must be taught by some professor 51

Practice n Professors have an SSN, a name, an age, a rank, and a research specialty. n Projects have a project number, a sponsor name (e. g. , NSF), a starting date, an ending date, and a budget. n Graduate students have an SSN, a name, an age, and a degree program n Each project is managed by exactly one professor (known as PI) n Each project is worked in by one or more professors (known as Co-PIs) n Each project is worked on by one or more graduate students (known as RAs) 52

n When graduate students work on a project, a professor must supervise their work on the project. Graduate students can work on multiple projects, in which case they will have a potentially different supervisor for each project. n Departments have a department number, a department name, and a main office. n Department has a professor (known as Chairman) who runs the department. 53

n Professors work in one or more departments, and for each department that they work in, a time percentage is associated with their job n Graduate students have one major department in which they are working on their degree. n Each graduate student must have another, more senior graduate student as an advisor. 54

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n A company database needs to store information about employees (identified by ssn, with salary and phone as attributes), departments (identified by dno, with dname and budget as attributes), and children of employees (with name and age as attributes).

Exercise n A company database needs to store information about employees (identified by ssn, with salary and phone as attributes), departments (identified by dno, with dname and budget as attributes), and children of employees (with name and age as attributes). Employees work in departments; each department is managed by an employee; a child must be identified uniquely by name when the parent (who is an employee; assume that only one parent works for the company) is known. n Draw an ER diagram that captures this information.

n Employees work in departments; n each department is managed by an employee;

n a child must be identified uniquely by name when the parent (who is an employee; assume that only one parent works for the company) is known.

Exercise n You set up a database company, Art. Base, that builds a product for art galleries. The core of this product is a database with a schema that captures all the information that galleries need to maintain. n Galleries keep information about artists, their names (which are unique), birthplaces, age, and style of art. For each piece of artwork, the artist, the year it was made, its unique title, its type of art (e. g. , painting, lithograph, sculpture, photograph), and its price must be stored. Pieces of artwork are also classified into groups of various kinds, for example, portraits, still lifes, works by Picasso, or works of the 19 th century; a given piece may belong to more than one group. Each group is identified by a name (like those just given) that describes the group. Finally, galleries keep information about customers. For each customer, galleries keep that person’s unique name, address, total amount of dollars spent in the gallery (very important!), and the artists and groups of art that the customer tends to like. Draw the ER diagram for the database

Galleries keep information about artists, their names (which are unique), • birthplaces, age, and style of art. For each piece of artwork, the artist, the year it was made, its unique title, its • type of art (e. g. , painting, lithograph, sculpture, photograph), and its price must be stored.

• Pieces of artwork are also classified into groups of various kinds, for example, portraits, still lifes, works by Picasso, or works of the 19 th century; a given piece may belong to more than one group. • Each group is identified by a name (like those just given) that describes the group.

• Finally, galleries keep information about customers. For each customer, galleries keep that person’s unique name, address, total amount of dollars spent in the gallery (very important!), • and the artists and groups of art that the customer tends to like

Subclasses Relations 1. Object-oriented: each entity is in one class. Create a relation for each class, with all the attributes for that class. 2. E/R style: an entity is in a network of classes related by isa. Create one relation for each E. S. n An entity is represented in the relation for each subclass to which it belongs. n Relation has only the attributes attached to that E. S. + key. 3. Use nulls. Create one relation for the root class or root E. S. , with all attributes found anywhere in its network of subclasses. n Put NULL in attributes not relevant to a given entity.

Example name Beers isa color Ales manf

OO-Style Beers Ales E/R Style Beers Ales Using NULLS Beers

Design Issues n Use of entity sets vs. attributes n Use of phone as an entity allows extra information about phone numbers (plus multiple phone numbers)

Design Issues n Use of entity sets vs. relationship sets Possible guideline is to designate a relationship set to describe an action that occurs between entities

Preview: Queries

• Read the following database case and then draw the EERD for that case. After that, transform that EERD into a relational model. Assume from 3 -5 attributes for each entity type. ABC is a company whose business is to deliver shipments. The company has many branches in Egypt each of which has many stores. Each store has many employees working for it. A store receives many customer shipments to deliver to destinations. A customer is able to send many shipments and for each he/she is expecting a confirmation on delivery. A fare is payable for each delivery. However, when the shipment is lost an apology is to replace the confirmation. ABC is using many vehicles including aircrafts, trucks and ships for sending the customer shipments.