Relational algebra in dbms
Relational algebra is a formal language for expressing operations on relational databases. It is used to manipulate and query data stored in tables, and it is the foundation of Structured Query Language (SQL), which is the most widely used language for working with relational databases.
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| DBMS Relational algebra skills arena |
Relational algebra consists of a set of operations that can be applied to one or more tables to produce a new table. Some common operations in relational algebra include:
Selection:
This operation selects a subset of rows from a table based on a given condition. For example, the selection operation could be used to select all rows where the value in the "Age" column is greater than 30.
SELECT * FROM Customers WHERE Age > 30
Projection:
This operation selects a subset of columns from a table. For example, the projection operation could be used to select only the "Name" and "Address" columns from a table.
SELECT Name, Address FROM Customers
Union:
This operation combines the rows of two tables into a single table. The two tables must have the same number of columns, and the columns must be of the same data type.
SELECT * FROM Customers UNION SELECT * FROM Employees
Intersection:
This operation returns only the rows that are present in both tables. The two tables must have the same number of columns, and the columns must be of the same data type.
SELECT * FROM Customers INTERSECT SELECT * FROM Employees
Difference:
This operation returns the rows that are present in the first table but not in the second. The two tables must have the same number of columns, and the columns must be of the same data type.
SELECT * FROM Customers MINUS SELECT * FROM Employees
Join:
This operation combines rows from two tables based on a common field. There are several types of joins, including inner join, outer join, and cross join. For example:
SELECT * FROM Customers INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
Here is an example of how these operations might be used in a relational algebra query:
SELECT * FROM Customers WHERE Age > 30 UNION SELECT * FROM Employees WHERE Age > 35
This query uses the selection and union operations to combine the rows from the "Customers" and "Employees" tables where the value in the "Age" column is greater than 30 and 35, respectively. The result is a new table that contains all of the rows from both tables that meet the specified condition.
Relational algebra is a powerful tool for manipulating and querying data in a relational database. It is used extensively in SQL, and it is an important part of any database developer's toolkit.
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Created by-- HARSH CHAUHAN
