Relational Algebra Overview

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Questions and Answers

What is the PROJECT operation denoted by?

π (pi) (correct)
σ (sigma)
ρ ( rho )
λ (lambda)

What does the PROJECT operation do to the columns of a relation?

It renames certain columns
It removes certain columns and keeps the others
It keeps certain columns and discards the others (correct)
It rearranges certain columns

What is the main purpose of the PROJECT operation?

To combine two relations
To remove duplicate tuples from a relation
To select specific columns from a relation (correct)
To reorder the columns of a relation

What is the result of applying the PROJECT operation to a relation?

A relation with certain columns removed (C) Signup and view all the answers

What is an alternative term for the PROJECT operation?

SELECT (D) Signup and view all the answers

What is a relation in a relational model based database?

A table in the database (B) Signup and view all the answers

What does the term 'relationship' refer to in a relational model based database?

How two tables are connected together (D) Signup and view all the answers

What is the main difference between relation and relationship in DBMS?

Table and connection between tables (C) Signup and view all the answers

What is the role of a relation in a relational model based database?

To store and manage data (A) Signup and view all the answers

What is the primary function of a relationship in a relational model based database?

To establish connections between tables (D) Signup and view all the answers

What can be said about the number of tuples in the result of projection π(R)?

It is always less than or equal to the number of tuples in R (C) Signup and view all the answers

What is a condition under which the number of tuples in the result of PROJECT is equal to the number of tuples in R?

If the list of attributes includes a key of R (D) Signup and view all the answers

Is the PROJECT operation commutative?

No, the PROJECT operation is not commutative (D) Signup and view all the answers

What is the condition under which π(π(R)) = π(R)?

π contains the attributes in (C) Signup and view all the answers

What is the purpose of the PROJECT operation?

To select certain attributes from a relation (C) Signup and view all the answers

What is a necessary condition for a union operation to be valid?

The relations have the same number of attributes. (D) Signup and view all the answers

What is a requirement for relations r and s to be union-compatible?

They must have the same number of attributes. (A) Signup and view all the answers

Which of the following is NOT a condition for a valid union operation?

The relations have the same database. (B) Signup and view all the answers

What is the primary purpose of checking the number of attributes in a union operation?

To ensure the relations have the same number of attributes. (B) Signup and view all the answers

Why do relations r and s need to have the same number of attributes for a valid union operation?

To ensure the union operation is valid. (D) Signup and view all the answers

What does the INTERSECTION operation (∩) produce?

A relation with tuples that are in both R and S (A) Signup and view all the answers

What is a requirement for the two operand relations R and S?

They must be type compatible (A) Signup and view all the answers

What are the attribute names in the result of the INTERSECTION operation?

The same as the attribute names in R (D) Signup and view all the answers

What is the symbol used to denote the INTERSECTION operation?

∩ (B) Signup and view all the answers

What is the result of R ∩ S?

A relation with tuples that are in both R and S (A) Signup and view all the answers

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Study Notes

Relational Algebra Overview

In a relational model-based database, a "relation" refers to a table, while a "relationship" refers to the connection between two tables.

Unary Relational Operations: PROJECT

The PROJECT operation is denoted by π (pi) and keeps certain columns (attributes) from a relation, discarding the others.
Properties of PROJECT operation:
- The number of tuples in the result of projection π(R) is always less than or equal to the number of tuples in R.
- If the list of attributes includes a key of R, then the number of tuples in the result of PROJECT is equal to the number of tuples in R.
- PROJECT is not commutative: π (π (R) ) = π (R) as long as contains the attributes in .

Relational Algebra Operations from Set Theory: UNION

For a union operation to be valid, the following conditions must hold:
- R and S must have the same number of attributes.

Relational Algebra Operations from Set Theory: INTERSECTION

INTERSECTION is denoted by ∩.
The result of the operation R ∩ S is a relation that includes all tuples that are in both R and S.
The attribute names in the result will be the same as the attribute names in R.
The two operand relations R and S must be "type compatible".