Linking data with JOIN

JOIN is used to link rows from two or more tables together. Hive supports most SQL JOIN operations, such as INNER JOIN and OUTER JOIN. In addition, HQL supports some special joins, such as MapJoin and Semi-Join too. In its earlier version, Hive only supported equal join. After v2.2.0, unequal join is also supported. However, you should be more careful when using unequal join unless you know what is expected, since unequal join is likely to return many rows by producing a Cartesian product of joined tables. When you want to restrict the output of a join, you should apply a WHERE clause after join as JOIN occurs before the WHERE clause. If possible, push filter conditions on the join conditions rather than where conditions to have data filtered earlier. What's more, all types of left/right joins are not commutative and always left/right associative, while...