Example

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©Silberschatz, Korth and Sudarsha 2.1 abase System Concepts - 5 th Edition, Oct 5, 2006 Example

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Example. Example. Example. Extended Relational-Algebra-Operations. Generalized Projection Aggregate Functions Outer Join. Generalized Projection. Extends the projection operation by allowing arithmetic functions to be used in the projection list. E is any relational-algebra expression - PowerPoint PPT Presentation

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Page 1: Example

©Silberschatz, Korth and Sudarshan2.1Database System Concepts - 5th Edition, Oct 5, 2006

Example

Page 2: Example

©Silberschatz, Korth and Sudarshan2.2Database System Concepts - 5th Edition, Oct 5, 2006

Example

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©Silberschatz, Korth and Sudarshan2.3Database System Concepts - 5th Edition, Oct 5, 2006

Example

Page 4: Example

©Silberschatz, Korth and Sudarshan2.4Database System Concepts - 5th Edition, Oct 5, 2006

Extended Relational-Algebra-Operations

Generalized Projection Aggregate Functions Outer Join

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©Silberschatz, Korth and Sudarshan2.5Database System Concepts - 5th Edition, Oct 5, 2006

Generalized Projection

Extends the projection operation by allowing arithmetic functions to be used in the projection list.

E is any relational-algebra expression

Each of F1, F2, …, Fn are are arithmetic expressions involving constants

and attributes in the schema of E. Given relation credit_info(customer_name, limit, credit_balance), find

how much more each person can spend:

customer_name, limit – credit_balance (credit_info)

)( ,...,,21

EnFFF

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©Silberschatz, Korth and Sudarshan2.6Database System Concepts - 5th Edition, Oct 5, 2006

Aggregate Functions and Operations

Aggregation function takes a collection of values and returns a single value as a result.

avg: average valuemin: minimum valuemax: maximum valuesum: sum of valuescount: number of values

Aggregate operation in relational algebra

E is any relational-algebra expression

Each Fi is an aggregate function

Each Ai is an attribute name

)()(,,(),( 2211E

nn AFAFAF

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©Silberschatz, Korth and Sudarshan2.7Database System Concepts - 5th Edition, Oct 5, 2006

Aggregate Operation – Example

Relation r:

A B

C

7

7

3

10

g sum(c) (r) sum(c )

27

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©Silberschatz, Korth and Sudarshan2.8Database System Concepts - 5th Edition, Oct 5, 2006

pt_works relation

employee_name branch_name salary

adams Mirpur 1500

Brown Mirpur 1300

johnson Dhanmondi 5300

gopal Dhanmondi 1700

Peterson Sadarghat 1600

Find out the total sum of salaries of all the part time employees in the bank.

g sum(salary) (pt_works)

Find out the total no. of branches appearing in the pt_works relation.

g count (distinct(branch_name)) (pt_works)

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©Silberschatz, Korth and Sudarshan2.9Database System Concepts - 5th Edition, Oct 5, 2006

Aggregate Functions and Operations

Aggregate operation in relational algebra using group

E is any relational-algebra expression

G1, G2 …, Gn is a list of attributes on which to group (can be empty)

Each Fi is an aggregate function

Each Ai is an attribute name

)()(,,(),(,,, 221121E

nnn AFAFAFGGG

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©Silberschatz, Korth and Sudarshan2.10Database System Concepts - 5th Edition, Oct 5, 2006

Aggregate Operation – Example

Relation account grouped by branch-name:

branch_name g sum(balance) (account)

branch_name account_number balance

PerryridgePerryridgeBrightonBrightonRedwood

A-102A-201A-217A-215A-222

400900750750700

branch_name sum(balance)

PerryridgeBrightonRedwood

13001500700

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©Silberschatz, Korth and Sudarshan2.11Database System Concepts - 5th Edition, Oct 5, 2006

Example query

Find out the maximum salary for part-time employees at each branch, in addition to the sum of salaries

branch_name g sum(salary), max(salary) (pt_works)

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©Silberschatz, Korth and Sudarshan2.12Database System Concepts - 5th Edition, Oct 5, 2006

Aggregate Functions (Cont.)

Result of aggregation does not have a name Can use rename operation to give it a name For convenience, we permit renaming as part of aggregate

operation

branch_name g sum(balance) as sum_balance (account)

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©Silberschatz, Korth and Sudarshan2.13Database System Concepts - 5th Edition, Oct 5, 2006

Example

branch (branch_name, branch_city, assets)

customer (customer_name, customer_street, customer_city)

account (account_number, branch_name, balance)

depositor (customer_name, account_number)

Find the accounts held by more than two customers using an aggregate

function.

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©Silberschatz, Korth and Sudarshan2.14Database System Concepts - 5th Edition, Oct 5, 2006

Example

Find the company with the most employees Find the company with the smallest payroll

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©Silberschatz, Korth and Sudarshan2.15Database System Concepts - 5th Edition, Oct 5, 2006

Example

Find the company with the most employees

Find the company with the smallest payroll


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