A semijoin is which of the following?

Solution (By Examveda Team)

A semijoin is a relational operation used in distributed database systems to reduce the amount of data transferred between sites while performing join operations.

Option A: Only the joining attributes are sent from one site to another and then all of the rows are returned.
This option describes a scenario where only the joining attributes are transmitted from one site to another, but all rows from the joined tables are returned. However, in a semijoin operation, only the required rows from one table (the outer table) are returned, not all rows.

Option B: All of the attributes are sent from one site to another and then only the required rows are returned.
This option suggests that all attributes from one site are transmitted to another, and then only the required rows are returned. However, in a semijoin operation, only the joining attributes are transmitted, not all attributes.

Option C: Only the joining attributes are sent from one site to another and then only the required rows are returned.
This option accurately describes a semijoin operation. In a semijoin, only the joining attributes from one table (the outer table) are sent to another site, where they are used to filter the rows of the other table (the inner table). Only the rows from the inner table that match the joining attributes are returned, reducing the amount of data transferred between sites.

Therefore, in the context of a semijoin operation in a distributed database system, the correct option is Only the joining attributes are sent from one site to another and then only the required rows are returned (Option C).