Types of Join Dependency

There are two types of Join Dependencies:

• Lossless Join Dependency: It means that whenever the join occurs between the tables, then no information should be lost, the new table must have all the content in the original table.
• Lossy Join Dependency: In this type of join dependency, data loss may occur at some point in time which includes the absence of a tuple from the original table or duplicate tuples within the database.

Join Dependency (JD) can be illustrated as when the relation R is equal to the join of the sub-relations R1, R2,…, and Rn are present in the database. Join Dependency arises when the attributes in one relation are dependent on attributes in another relation, which means certain rows will exist in the table if there is the same row in another table. Multiple tables are joined to create a single table where one of the attributes is common in the sub-tables. We can also relate the join dependency to the 5^th Normal Form. A join dependency is said to be not that important if any relational schemas in the join dependency are equivalent to the original relation R.

Join dependency on a database is denoted by:

R₁ ⨝ R₂ ⨝ R₃ ⨝ ….. ⨝ R_{n ;}

where R_{1 ,} R_2, … _, R_n are the relations and ⨝ represents the natural join operator.