SQL LEFT JOIN, also known as LEFT OUTER JOIN, is a type of join operation used in relational databases to combine data from two tables based on a common column or key. The result set of a LEFT JOIN includes all the rows from the left table and the matching rows from the right table, as well as any unmatched rows from the left table.
Here’s the syntax for a LEFT JOIN:
SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;
In this syntax, table1 is the left table, and table2 is the right table. The ON keyword specifies the join condition, which is used to match rows between the two tables based on a common column or key.
The result of a LEFT JOIN includes all the rows from table1 and the matching rows from table2, but it also includes any unmatched rows from table1. If there is no matching row in table2 for a particular row in table1, the columns from table2 will contain NULL values in the result set.
Here’s an example to illustrate the usage of LEFT JOIN:
SELECT Customers.CustomerID, Customers.CustomerName, Orders.OrderID FROM Customers LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID ORDER BY Customers.CustomerName;
In this example, we’re joining the Customers and Orders tables on the CustomerID column. The result set will include all the customers from the Customers table, along with any matching orders from the Orders table. If a customer doesn’t have any orders, the OrderID column will contain NULL values in the result set.
LEFT JOIN is a powerful tool in SQL that allows you to combine data from multiple tables and perform complex queries. It’s important to understand how to use it effectively to get the most out of your data.