Huiswerk
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Transcript of Huiswerk
©Silberschatz, Korth and Sudarshan2.1Database System Concepts
HuiswerkHuiswerk
Lees delen 3.2, 3.3 van hoofdstuk 3.
opgaven voor hoofdstuk 2: modelleeropgave 5
opgaven voor hoofdstuk 3: maak de queries voor de vragen uit 3.5 in relationele algebra; maak de queries 1-6 voor de bierdrinkerdatabase in tuppel calculus EN in relationele algebra.
©Silberschatz, Korth and Sudarshan2.2Database System Concepts
Silberschatz 3.5 aSilberschatz 3.5 a
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Find the names of all employees who work for FBC
{ t | w works ( t[person-name] =w[person-name] Λ
w[company-name]=“FBC” )}
©Silberschatz, Korth and Sudarshan2.3Database System Concepts
Silberschatz 3.5 bSilberschatz 3.5 b
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Find the names and the cities of residence of all employees who work for FBC
{ t | e employee (t[person-name] = e[person-name] Λ t[city] = e[city]
Λ w works ( w[person-name] =e[person-name]
Λ w[company-name]=“FBC”))}
©Silberschatz, Korth and Sudarshan2.4Database System Concepts
Silberschatz 3.5 cSilberschatz 3.5 c
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Find the names, street address, and the cities of residence of all employees who work for First Bank Corporation and earn more that $10000 per annum.
{ t | e employee (t[person-name] = e[person-name] Λ t[city] = e[city] Λ t[street] = e[street] Λ
w works ( w[person-name] =e[person-name] Λ
w[company-name]=“FBC” Λ
w[salary] >10000))}
{ t | t employee Λ w works ( w[person-name] =t[person-name] Λ
w[company-name]=“FBC” Λ
w[salary] >10000))}
©Silberschatz, Korth and Sudarshan2.5Database System Concepts
Silberschatz 3.5 dSilberschatz 3.5 d
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Find the names of all employees who live in the same city as the company for which they work.
{ t | w works ( t[person-name] =w[person-name] Λ ccompany (
c[company-name=w[company-name] Λ
eemployee ( e[person-name] = w[person-name] Λ
e[city] = c[city] )))}
©Silberschatz, Korth and Sudarshan2.6Database System Concepts
Silberschatz 3.5 eSilberschatz 3.5 e
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Find the names of all employees who live in the same city and on the same street as do their managers.
{ t | eemployee (t[person-name] = e[person-name] Λ
mmanages ( m[person-name] = e[person-name] Λ
em employee (em[person-name] = m[person-name] Λ
e[city] = em[city] Λ e[street] = em[street] )))}
©Silberschatz, Korth and Sudarshan2.7Database System Concepts
Silberschatz 3.5 fSilberschatz 3.5 f
employee (person-name, street, city) works (person-name, company-name, salary) company (company-name, city) manages (person-name, manager_name)
Find the names of all employees in this database who do not work for FBC.
single company assumption
{ t | wworks (t[person-name]=w[person-name] Λ [company-name]≠“FBC”)} Multiple company assumption
{ t | wworks (t[person-name]=w[person-name] Λ
w1 works (w[person-name] = w1[person-name]
w1[company-name]≠“FBC”))} Which assumption holds according to the definition of the database?
©Silberschatz, Korth and Sudarshan2.8Database System Concepts
Silberschatz 3.5 gSilberschatz 3.5 g
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Find the names of all employees who earn more than every employee of SBC.
{ t | wworks (t[person-name]=w[person-name] Λ
w1 works (w1[company-name] = “SBC” w[salary] > w1[salary] ))}
©Silberschatz, Korth and Sudarshan2.9Database System Concepts
Silberschatz 3.5 hSilberschatz 3.5 h
employee (person-name, street, city)
works (person-name, company-name, salary)
company (company-name, city)
manages (person-name, manager_name)
Assume the companies may be located in several cities. Find all companies located in every city in which SBC is located.
{ t | ccompany (t[company-name]=c[company-name] Λ
c1 company (c1[company-name] = “SBC”
c2company (c2[company-name]=c[company-name] Λ c2[city] = c1[city] )))}
©Silberschatz, Korth and Sudarshan2.10Database System Concepts
Reduction of an E-R Schema to TablesReduction of an E-R Schema to Tables
Primary keys allow entity sets and relationship sets to be expressed uniformly as tables which represent the contents of the database.
A database which conforms to an E-R diagram can be represented by a collection of tables.
For each entity set and relationship set there is a unique table which is assigned the name of the corresponding entity set or relationship set.
Each table has a number of columns (generally corresponding to attributes), which have unique names.
Converting an E-R diagram to a table format is the basis for deriving a relational database design from an E-R diagram.
©Silberschatz, Korth and Sudarshan2.11Database System Concepts
Representing Entity Sets as TablesRepresenting Entity Sets as Tables
A strong entity set reduces to a table with the same attributes.
©Silberschatz, Korth and Sudarshan2.12Database System Concepts
Composite and Multivalued AttributesComposite and Multivalued Attributes
Composite attributes are flattened out by creating a separate attribute for each component attribute E.g. given entity set customer with composite attribute name with
component attributes first-name and last-name the table corresponding to the entity set has two attributes name.first-name and name.last-name
A multivalued attribute M of an entity E is represented by a separate table EM Table EM has attributes corresponding to the primary key of E and an
attribute corresponding to multivalued attribute M E.g. Multivalued attribute dependent-names of employee is represented
by a table employee-dependent-names( employee-id, dname)
Each value of the multivalued attribute maps to a separate row of the table EM E.g., an employee entity with primary key John and
dependents Johnson and Johndotir maps to two rows: (John, Johnson) and (John, Johndotir)
©Silberschatz, Korth and Sudarshan2.13Database System Concepts
Representing Weak Entity SetsRepresenting Weak Entity Sets
A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set
©Silberschatz, Korth and Sudarshan2.14Database System Concepts
Representing Relationship Sets as Representing Relationship Sets as TablesTables
A many-to-many relationship set is represented as a table with columns for the primary keys of the two participating entity sets, and any descriptive attributes of the relationship set.
E.g.: table for relationship set borrower
©Silberschatz, Korth and Sudarshan2.15Database System Concepts
Redundancy of TablesRedundancy of Tables
Many-to-one and one-to-many relationship sets that are total on the many-side can be represented by adding an extra attribute to the many side, containing the primary key of the one side
E.g.: Instead of creating a table for relationship account-branch, add an attribute branch to the entity set account
©Silberschatz, Korth and Sudarshan2.16Database System Concepts
Redundancy of Tables (Cont.)Redundancy of Tables (Cont.)
For one-to-one relationship sets, either side can be chosen to act as the “many” side That is, extra attribute can be added to either of the tables
corresponding to the two entity sets
If participation is partial on the many side, replacing a table by an extra attribute in the relation corresponding to the “many” side could result in null values
The table corresponding to a relationship set linking a weak entity set to its identifying strong entity set is redundant. E.g. The payment table already contains the information that would
appear in the loan-payment table (i.e., the columns loan-number and payment-number).
©Silberschatz, Korth and Sudarshan2.17Database System Concepts
Representing Specialization as TablesRepresenting Specialization as Tables Method 1:
Form a table for the higher level entity
Form a table for each lower level entity set, include primary key of higher level entity set and local attributes
table table attributesperson name, street, city customer name, credit-ratingemployee name, salary
Drawback: getting information about, e.g., employee requires accessing two tables
©Silberschatz, Korth and Sudarshan2.18Database System Concepts
Representing Specialization as Tables Representing Specialization as Tables (Cont.)(Cont.)
Method 2: Form a table for each entity set with all local and inherited
attributestable table attributes
person name, street, citycustomer name, street, city, credit-ratingemployee name, street, city, salary
If specialization is total, table for generalized entity (person) not required to store information Can be defined as a “view” relation containing union of
specialization tables But explicit table may still be needed for foreign key constraints
Drawback: street and city may be stored redundantly for persons who are both customers and employees
©Silberschatz, Korth and Sudarshan2.19Database System Concepts
Relations Corresponding to Relations Corresponding to AggregationAggregation
To represent aggregation, create a table containing
primary key of the aggregated relationship,
the primary key of the associated entity set
Any descriptive attributes
©Silberschatz, Korth and Sudarshan2.20Database System Concepts
Relations Corresponding to Relations Corresponding to Aggregation (Cont.)Aggregation (Cont.)
E.g. to represent aggregation manages between relationship works-on and entity set manager, create a table manages(employee-id, branch-name, title, manager-name)
Table works-on is redundant provided we are willing to store null values for attribute manager-name in table manages
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ExampleExample
©Silberschatz, Korth and Sudarshan2.22Database System Concepts
Translate into the relational modelTranslate into the relational model