When using just JOIN in a SQL statement, it is meant to be an INNER JOIN. Joining tables based on primary-foreign key relationships is fundamental to RDBMSs and is important to Codd's relational model. SELF JOIN, CROSS JOIN, LEFT OUTER JOIN, INNER JOIN is just a few examples. Joins are used to combine rows of two or more tables based on key pairs and data returned can be reduced based on predicates and clauses towards the end of statements.