1. sql
  2. /joins
  3. /left-join

SQL LEFT JOIN Keyword

Definition

The LEFT JOIN keyword is used in SQL to combine two or more tables together, with the goal of returning all rows from the left table (also known as the first table), and any matching rows from the right table (also known as the second table). If there is no match, the result will contain NULL values for all columns of the right table.

Example

Here's an example of how you might use the LEFT JOIN keyword in a SQL query:

SELECT orders.id, customers.name 
FROM orders 
LEFT JOIN customers 
ON orders.customer_id = customers.id;

In this example, the query is selecting the id and name columns from the orders and customers tables, respectively. The ON clause specifies the relationship between the two tables, in this case, the customer_id column of the orders table is matched with the id column of the customers table. This query will return all rows from the orders table, along with the corresponding name from the customers table for each order. If there is no match in the customers table, it will return a NULL value for the name column.

LEFT JOIN is similar to INNER JOIN but the INNER JOIN returns only the matching rows from both tables, where as LEFT JOIN returns all the rows of left table along with matching rows of the right table.

It's worth noting that you can use the LEFT JOIN keyword with more than two tables, as well as other types of joins such as RIGHT JOIN and FULL OUTER JOIN.

Best Practices

Here are some best practices for using LEFT JOIN in SQL:

  • Use LEFT JOIN when you want to include unmatched rows from the left table in the result set. If you use a regular (INNER) JOIN, unmatched rows from the left table will be excluded from the result set.

  • Be sure to use the correct join condition in the ON clause. The join condition should match the related column(s) between the left and right tables.

  • Keep in mind that LEFT JOIN can return duplicate rows if there are multiple matches in the right table. Use the DISTINCT keyword or a GROUP BY clause to eliminate duplicates if needed.

  • Be aware that LEFT JOIN can be very slow if the right table is large, especially if it doesn't have an index on the join column(s). To improve performance, create an index on the joined column(s) in the right table.

  • Keep in mind that the use of LEFT JOIN on multiple tables can make the query more complex and harder to understand. It's a good idea to break them down into smaller queries, and then use the results of those queries to join with other tables.