Relational Algebra Flashcards

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Questions and Answers

Relational algebra is made up of which of the following five fundamental operations?

Difference (correct)
Union (correct)
Selection (correct)
Projection (correct)
Cartesian product (correct)

Which of the following should you use to ensure that union-compatibility exists between two relations from which you want to create a third relation that does not contain duplicate information?

Projection (correct)
Selection
Difference
Join

Which of the following SQL statements should you use to restrict a Cartesian product relational algebraic operation to avoid the redundancy of creating a relation that contains every possible row combination from two relations?

SELECT * FROM table1 UNION table2
SELECT * FROM table1, table2 WHERE condition (correct)
SELECT * FROM table1 WHERE condition
SELECT * FROM table1 JOIN table2

What is the term for a binary operation that combines two relations where the combination of rows satisfies a predicate?

Theta-join Signup and view all the answers

Consider the following relational algebraic expression: σ location = 'BldgA' (Employee). Which SQL expression is equivalent to this relational algebraic expression?

SELECT * FROM Employee WHERE location = 'BldgA'; Signup and view all the answers

To which of the following is a join operation equivalent?

A restricted Cartesian product operation Signup and view all the answers

What do the relations known as that require the two relations involved to have the same number of attributes with matching domains?

Union-compatible relations Signup and view all the answers

What function does the intersection relational algebraic operation perform?

It creates a new relation from two relations containing all the rows that are in both the first and the second relations. Signup and view all the answers

Consider the following SQL statement: SELECT Proj_Mgr., Contractor. FROM Proj_Mgr, Contractor WHERE Proj_Mgr.e_id = Contractor.e_id. What type of operation is represented by the SQL statement?

Intersection Signup and view all the answers

Selection is a unary operation that:

uses a condition to select rows from a single relation to create a new relation. Signup and view all the answers

What function does the union relational algebraic operation perform?

It combines all rows from two relations into a single relation while eliminating any duplicates. Signup and view all the answers

Projection is a unary operation that:

uses a condition to select columns from a single relation to create a new relation. Signup and view all the answers

Which relational algebraic operation creates a new relation containing every possible pair of rows from the two original relations?

Cartesian product Signup and view all the answers

Consider the following relational algebraic expression: Employee ∪ Proj_Mgr. Which SQL statement is equivalent?

SELECT * FROM Employee UNION SELECT * FROM Proj_Mgr; Signup and view all the answers

You have two relations from which you can extract data: Employee and Proj_Mgr. Which relational algebraic operation should you use to determine which employees are not project managers?

Difference Signup and view all the answers

You have two relations from which you can extract data: Proj_Mgr and Contractor. Which relational algebraic operation should you use to determine the project managers who are contractors?

Intersection Signup and view all the answers

Which relational algebraic operation is represented by the symbol π?

Projection Signup and view all the answers

Consider the following SQL statement: SELECT Customer.c_id, last_name, first_name, address, city, state, zip, Tour.c_id, tour_no FROM Customer, Tour WHERE Customer.c_id = Tour.c_id. What type of operation is represented by this SQL statement?

An equi-join operation Signup and view all the answers

Consider the SQL statement: SELECT * FROM Employee WHERE hire_date = '06-01-2004'; Which relational algebraic expression is equivalent?

σ hire_date = '06-01-2004'(Employee) Signup and view all the answers

What distinguishes an equi-join from a theta-join?

An equi-join's predicate contains only the equality operator, whereas a theta-join's predicate can use any comparison operator. Signup and view all the answers

Consider the following relational algebraic expression: Employee X Project. Which SQL statement is equivalent to this relational algebraic expression?

SELECT Employee., Project. FROM Employee, Project; Signup and view all the answers

Which symbol indicates a Cartesian product relational algebraic operation?

X Signup and view all the answers

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Study Notes

Fundamental Operations of Relational Algebra

Relational algebra consists of five primary operations: selection, projection, Cartesian product, union, and difference.

Ensuring Union-Compatibility

To create a third relation without duplicates from two relations, utilize the projection operation.

Restricting Cartesian Products in SQL

The WHERE clause is essential to limit Cartesian products and reduce redundancy by restricting row combinations.

Theta-Join Definition

A theta-join is a binary operation that combines two relations based on a predicate satisfied by the rows.

Equivalent SQL Expressions for Selection

The expression σ location = 'BldgA' (Employee) corresponds to SQL: SELECT * FROM Employee WHERE location = 'BldgA';.

Equivalence of Join Operations

A join operation is equivalent to a restricted Cartesian product, combining rows based on specific conditions.

Union-Compatible Relations

Relations that must have the same number of attributes with matching domains are known as union-compatible relations.

Function of Intersection Operation

The intersection operation yields a new relation containing rows common to both input relations.

SQL Representation of Intersection

The SQL statement SELECT Proj_Mgr., Contractor. FROM Proj_Mgr, Contractor WHERE Proj_Mgr.e_id = Contractor.e_id represents an intersection of two relations.

Selection Operation Characteristics

Selection is a unary operation that selects rows from a single relation based on a specified condition.

Function of Union Operation

The union operation merges rows from two relations into one, ensuring the elimination of duplicate rows.

Characteristics of Projection Operation

Projection is a unary operation that chooses specific columns from a single relation to produce a new relation.

Cartesian Product Definition

The Cartesian product is a binary operation generating a new relation with every possible pair of rows from two relations.

Equivalent SQL for Union

The expression Employee ∪ Proj_Mgr corresponds to SQL: SELECT * FROM Employee UNION SELECT * FROM Proj_Mgr;.

Identifying Non-Project Managers

To find employees who are not project managers, employ the difference operation.

Finding Common Project Managers

The intersection operation is used to identify project managers who are also contractors.

Projection Symbol

In relational algebra, projection is denoted using the symbol π.

SQL Representation of Equi-Join

The SQL statement SELECT Customer.c_id, last_name, first_name, address, city, state, zip, Tour.c_id, tour_no FROM Customer, Tour WHERE Customer.c_id = Tour.c_id represents an equi-join operation.

Equivalent Expression for Specific SQL Queries

The SQL statement SELECT * FROM Employee WHERE hire_date = '06-01-2004' is equivalent to the relational algebra expression σ hire_date = '06-01-2004'(Employee).

Distinction Between Equi-Join and Theta-Join

An equi-join uses only the equality operator in its predicate, while a theta-join can utilize any comparison operator.

SQL Equivalent for Cartesian Products

The expression Employee X Project corresponds to SQL: SELECT Employee., Project. FROM Employee, Project.

Symbol for Cartesian Product

In relational algebra, the symbol X signifies the operation of Cartesian product.

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