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Left Join - (PostgreSQL Join)

A JOIN clause is used to combine rows from two or more tables based on a related column between them. A left join returns all the rows from the left table, along with any matching rows from the right table. In this tutorial, we'll show you how to use a left join in PostgreSQL.

Syntax

SELECT column_list
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name
  • column_list: A comma-separated list of columns to be selected from the tables.
  • table1: The left table in the join operation.
  • table2: The right table in the join operation.
  • column_name: The column that is common to both tables.

Example

Let's take a look at an example of using a left join in PostgreSQL.

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

When you execute this query, you will get all rows from the customers table, along with any matching rows from the orders table.

Explanation

In this example, we used a left join to combine data from the customers and orders tables. We selected the customer_id, customer_name, order_id and order_date columns from the tables.

The LEFT JOIN keyword returns all rows from the customers table, along with any matching rows from the orders table. If there is no matching row in the orders table, NULL values are returned for the columns selected from that table.

Use

Using a left join in PostgreSQL allows you to combine data from two or more tables based on a related column between them. This can be useful when you want to retrieve data from one table, even if there is no matching data in another table.

Important Points

  • When using a left join, the order of the tables in the join clause matters. The left table is always the first table.
  • If there is no matching row in the right table, NULL values are returned for the columns selected from that table.
  • If there are multiple matching rows in the right table, each matching row is returned as a separate row in the result set.

Summary

In this tutorial, we discussed how to use a left join in PostgreSQL. We covered the syntax, example, output, explanation, use, and important points when using a left join. With this knowledge, you can now use a left join in your PostgreSQL queries to combine data from two or more tables based on a related column between them.

INNER JoinRight Join
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