# Retrieve the names of authors who have written at least two books relational algebra

Aug 30, 2022

select author
from author
where count(author)>=2

## Other related questions:

### What are the two types of operators in relational algebra?

There are two types of operators in relational algebra: selection and projection.

### How many relational algebra operators are there?

There are eight basic relational algebra operators: select, project, union, intersect, difference, cross join, natural join, and outer join.

### What is relational algebra explain with example?

Relational algebra is a formal language for representing and manipulating data in relational databases. It is similar to first-order logic and has a well-defined syntax and semantics.

Relational algebra is used in database theory and practice for several reasons:

1. It is a simple and elegant way to specify queries.

2. It can be used to optimize queries.

3. It can be used to specify integrity constraints.

4. It provides a theoretical foundation for relational databases.

Example:

Consider the following relational database:

Students(sid: integer, name: string, age: integer, gpa: real)

Courses(cid: integer, name: string, credits: integer)

Enrolled(sid: integer, cid: integer, grade: string)

The following are examples of queries that can be expressed in relational algebra:

1. Find the names of all students who are enrolled in a course with the name “Database”.

2. Find the names of all students who are enrolled in a course with the name “Database” and have a grade of “A”.

3. Find the names of

### Who invented relational algebra?

There is no one definitive answer to this question as relational algebra is a field of mathematics with a long history. However, it is generally agreed that the earliest work on relational algebra was done by mathematicians such as Gottfried Wilhelm Leibniz and Giuseppe Peano in the late 19th and early 20th centuries.