In the world of database management, queries are the code that is used to access and retrieve data from a database. They are essential in building efficient and effective database systems. One type of query that is commonly used is the JOIN operation. The JOIN operation combines data from two or more tables in a database by matching rows between the tables based on a specified condition. There are different types of JOIN operators that are commonly used, including Left Join and Left Outer Join.
In this article, we’ll explain Left Join and Left Outer Join and their differences. You’ll also learn how to use them in a database management system. Additionally, we’ll provide examples to demonstrate how to use the JOIN operators in SQL queries.
What is a Left Join?
A LEFT JOIN, also referred to as a LEFT OUTER JOIN, is a JOIN operation that returns all the rows from the first table (the left table) and the matching rows from the second table (the right table). Any unmatched rows from the second table are returned as NULL values.
In simple terms, a LEFT JOIN selects all the rows from the left table and the matching rows from the right table. If there are no matching rows in the right table, the result set contains NULL values.
The syntax for performing a LEFT JOIN is as follows:
SELECT column_list
FROM table1
LEFT JOIN table2 ON join_condition;
Here, column_list refers to the list of columns to be selected from the table. table1 is the first table, and table2 is the second table to be joined. join_condition is the condition used to join the tables based on column values.
What is a Left Outer Join?
A Left Outer Join, also known as a Left Join, is the same as the Left Join. The term “Outer” is used to indicate that it includes all the rows from the left table that may not have a matching row in the right table.
When using a Left Outer Join, all the rows from the left table are returned, including those that have no matching rows in the right table. The NULL value is returned for columns in the right table that don’t have a matching row.
The syntax for performing a Left Outer Join is similar to the Left Join:
SELECT column_list
FROM table1
LEFT OUTER JOIN table2 ON join_condition;
Here, column_list, table1, and join_condition are the same as in the Left Join query.
Differences between Left Join and Left Outer Join
The only difference between a Left Join and a Left Outer Join is the inclusion of all rows from the left table in the result set. A Left Join only returns matching rows from the right table, while a Left Outer Join returns all rows from the left table and matching rows from the right table.
When to use a Left Join and a Left Outer Join
The choice between using a Left Join and a Left Outer Join depends on the requirements of the query.
A Left Join is useful when you only need to retrieve matching rows from both tables. It’s commonly used when you want to find all orders placed by customers, even if they haven’t placed any orders.
On the other hand, a Left Outer Join is useful when you want to retrieve all rows from the left table, regardless of whether they have matching rows in the right table or not. It’s commonly used when you want to retrieve all customers, even if they haven’t placed any orders.
Examples of Left Join and Left Outer Join
Suppose we have two tables in our database, Customers, and Orders. The Customers table has the following columns:
CustomerID
CustomerName
ContactName
Country
The Orders table has the following columns:
OrderID
CustomerID
OrderDate
ShipperName
The following are examples of queries that use Left Join and Left Outer Join:
Query 1: Find all customers and their orders
To find all customers and their orders, we can use a Left Outer Join query as follows:
SELECT C.CustomerName, O.OrderID, O.OrderDate
FROM Customers C
LEFT OUTER JOIN Orders O ON C.CustomerID = O.CustomerID
ORDER BY C.CustomerName;
This query retrieves all customer names from the Customers table, regardless of whether they have any matching rows in the Orders table. It also returns the order details from the Orders table for those customers who have placed orders.
Query 2: Find all orders and their customers
To find all orders and the customers who placed them, we can use a Left Join query as follows:
SELECT O.OrderID, C.CustomerName, C.Country
FROM Orders O
LEFT JOIN Customers C ON O.CustomerID = C.CustomerID
ORDER BY O.OrderID;
This query retrieves all orders from the Orders table, including those that don’t have any matching rows in the Customers table. It also returns the customer details from the Customers table for those orders that have customer information.
Conclusion
Left Join and Left Outer Join are types of JOIN operations used to combine data from two or more tables in a database. They’re used to retrieve data based on a specified condition.
A Left Join retrieves all matching rows from the right table but only returns the rows from the left table that match the condition. A Left Outer Join retrieves all rows from the left table, including those that don’t have matching rows in the right table.
When using JOIN operations, it’s essential to understand the requirements of the query to choose the right type of Join. Examples of Left Join and Left Outer Join queries can be created in a database management system using SQL.