The purpose of a self join in a database is option 1) To combine rows from the same table based on a related column.
A self join is a type of join operation in a relational database that involves joining a table to itself. This can be useful when working with hierarchical data or when you need to compare rows within the same table based on a related column.
The primary purpose of a self join is to combine rows from the same table based on a related column. This allows you to create a logical relationship between rows within the same table, typically using a foreign key that references the same table.
Self joins are commonly used to compare rows with similar attributes, such as in organizational structures where an employee table may have a "supervisor" column that references other employees within the same table. By performing a self join, you can retrieve information about the supervisors and their subordinates from the same table.
In conclusion, a self join in a database is specifically designed to combine rows from the same table based on a related column. It's a valuable tool for retrieving and analyzing hierarchical or related data within a single table, allowing for a wide range of practical applications in database querying and analysis.