Question

Difference between selection and projection in relational algebra

Answer 1

Answer:

Explanation:

In Relational algebra 'Selection' and 'Projection' are different operations, but the SQL SELECT combines these operations in a single statement. Select retrieves the tuples (rows) in a relation (table) for which the condition in 'predicate' section (WHERE clause) stands true.

Answer 2

The difference between selection and projection is Selection means which rows are to be returned and Projection means choosing which columns the query shall return.

Explanation:

• Relational Algebra gives output based on the input command and it is performed recursively.

• Selection is also used to select a row also known as the tuple from a relation based on the condition.

• The condition is also known as the predicate.

• It is denoted by $\sigma$ (Sigma).

• Example:  For selecting  rows from Student relation having age of 18 years. It is given by $\sigma$ (age = 18 ) (Student)

• Projection is used to select some attribute from a relation , depending upon the Predicate.

• Example: For selecting consumer name from Customer relation having age greater than 20.

• It is written as  $\pi$ (CustomerName, age>20)  (Customers)  

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