Understanding the joins concept

A join in Hive is used for the same purpose as in a traditional RDBMS. A join is used to fetch meaningful data from two or more tables based on a common value or field. In other words, a join is used to combine data from multiple tables. A join is performed whenever multiple tables are specified inside the FROM clause.

As of now, joins based on equality conditions only are supported in Hive. It does not support any join condition that is based on non-equality conditions.

The general syntax of defining a join is as follows:

join_table:
    table_reference JOIN table_factor [join_condition]
  | table_reference {LEFT|RIGHT|FULL} [OUTER] JOIN table_reference    join_condition
  | table_reference LEFT SEMI JOIN table_reference join_condition
  | table_reference CROSS JOIN table_reference [join_condition] 

table_reference:
    table_factor
  | join_table

  table_factor:
    tbl_name [alias]
  | table_subquery alias
  | ( table_references )

join_condition:
   ...