How is ‘join’ used in a hive?

2 years ago
Noah Thompson
2 minutes

In Hive, JOIN is used to connect two or more tables based on one or more common columns. JOIN operation allows users to retrieve related data from multiple tables.

The syntax for JOIN in Hive is as follows:

SELECT <columns>
FROM <table1>
JOIN <table2> ON <table1.column = table2.column>
[JOIN <table3> ON <table1.column = table3.column>]
...

In this case, refers to the list of columns to be retrieved, , , are the names of the tables to be joined, and are the columns to be joined.

There are various types of JOIN operations, such as:

  1. INNER JOIN: retrieves rows from both tables that have a match.
  2. LEFT JOIN: Returns all rows from the left table and the matched rows from the right table.
  3. RIGHT JOIN: Returns all rows from the right table and the matching rows from the left table.
  4. FULL OUTER JOIN: Returns all rows from both the left and right tables.

JOIN operation can also involve multiple tables, known as multi-table JOIN. In a multi-table JOIN, each table needs to have common columns with the previous connected table.

For example, the following query will return rows that match in the tables orders and customers:

SELECT orders.order_id, customers.customer_name
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id;

This will return rows that have the same customer_id in both the orders table and the customers table, selecting the order_id column from the orders table and the customer_name column from the customers table.

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