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?

    <p>Theta-join</p> 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?

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

    To which of the following is a join operation equivalent?

    <p>A restricted Cartesian product operation</p> 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?

    <p>Union-compatible relations</p> Signup and view all the answers

    What function does the intersection relational algebraic operation perform?

    <p>It creates a new relation from two relations containing all the rows that are in both the first and the second relations.</p> 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?

    <p>Intersection</p> Signup and view all the answers

    Selection is a unary operation that:

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

    What function does the union relational algebraic operation perform?

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

    Projection is a unary operation that:

    <p>uses a condition to select columns from a single relation to create a new relation.</p> 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?

    <p>Cartesian product</p> Signup and view all the answers

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

    <p>SELECT * FROM Employee UNION SELECT * FROM Proj_Mgr;</p> 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?

    <p>Difference</p> 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?

    <p>Intersection</p> Signup and view all the answers

    Which relational algebraic operation is represented by the symbol π?

    <p>Projection</p> 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?

    <p>An equi-join operation</p> 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?

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

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

    <p>An equi-join's predicate contains only the equality operator, whereas a theta-join's predicate can use any comparison operator.</p> 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?

    <p>SELECT Employee.<em>, Project.</em> FROM Employee, Project;</p> Signup and view all the answers

    Which symbol indicates a Cartesian product relational algebraic operation?

    <p>X</p> Signup and view all the answers

    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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    Test your knowledge of relational algebra with these flashcards. Each card covers fundamental operations and concepts essential for understanding relational databases. Perfect for students and professionals alike!

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