Understanding Left Outer Join in SQL Server

Greetings, Dev! If you are working with SQL Server, you might come across a situation where you need to combine data from two or more tables. In such situations, you can use various types of SQL joins. In this article, we will focus on one of the most commonly used joins, which is the left outer join. We will explain what a left outer join is, how it works, and how you can use it in your SQL Server queries. So, let’s dive in!

What is a Left Outer Join?

A left outer join is a type of join where all the rows from the left table (also known as the “preserved” table) and matching rows from the right table are returned. In case there are no matching rows in the right table, the result set will contain NULL values for the right table’s columns.

Let’s say you have two tables, “Customers” and “Orders”, and you want to retrieve all the customers even if they have not placed any orders. You can use a left outer join to achieve this.

Customers	Orders
CustomerID	OrderID
CustomerName	OrderDate
Address	CustomerID
City	Amount

How Does a Left Outer Join Work?

When you use a left outer join, the SQL engine first selects all the rows from the left table, and then tries to find matching rows from the right table based on the join condition. The join condition specifies the columns that are used to match the rows between the two tables.

If a matching row is found in the right table, the SQL engine combines the left and right table rows and returns the resulting row. If no matching row is found in the right table, the SQL engine returns NULL values for the right table’s columns.

Let’s see an example to better understand how a left outer join works in SQL Server.

Example:

Suppose we have two tables, “Employees” and “Departments”, as shown in the following figure:

Employees	Departments
EmpID	DeptID
FirstName	DeptName
LastName	Location
Salary

Suppose we want to retrieve all the employees and their respective departments. However, some employees may not be assigned to any department yet. In this scenario, we can use a left outer join.

The SQL statement for the left outer join would look like this:

SELECT e.FirstName, e.LastName, d.DeptNameFROM Employees eLEFT OUTER JOIN Departments d ON e.DeptID = d.DeptID;

The result set of the query would contain all the employees from the “Employees” table and their respective departments, if they have any. If an employee is not assigned to any department yet, the department name would be NULL.

Now that we have seen how a left outer join works, let’s see some frequently asked questions related to left outer join in SQL Server.

FAQs

Q1: What is the difference between left outer join and inner join?

A: In a left outer join, all the rows from the left table (preserved table) are returned along with matching rows from the right table. In case there are no matching rows in the right table, NULL values are returned for the right table’s columns. In an inner join, only the matching rows from both tables are returned.

Q2: How do I write a left outer join query in SQL Server?

A: The syntax for a left outer join query in SQL Server is as follows:

SELECT columnsFROM table1LEFT OUTER JOIN table2 ON join_condition;

Replace “columns” with the columns you want to retrieve, “table1” with the left table, “table2” with the right table, and “join_condition” with the condition used to match the rows between two tables.

Q3: Can I use multiple left outer joins in a single SQL query?

A: Yes, you can use multiple left outer joins in a single SQL query. The syntax of the query would be as follows:

SELECT columnsFROM table1LEFT OUTER JOIN table2 ON join_condition1LEFT OUTER JOIN table3 ON join_condition2;

You can continue adding left outer joins to the query as required.

Q4: Can I use left outer join with more than two tables?

A: Yes, you can use left outer join with more than two tables. The syntax of the query would be as follows:

SELECT columnsFROM table1LEFT OUTER JOIN table2 ON join_condition1LEFT OUTER JOIN table3 ON join_condition2LEFT OUTER JOIN table4 ON join_condition3;

You can continue adding left outer joins to the query as required.

Q5: Can I use left outer join with other types of joins?

A: Yes, you can use left outer join with other types of joins such as inner join, right outer join, and full outer join. These are known as combined joins. The syntax of the query would be as follows:

SELECT columnsFROM table1LEFT OUTER JOIN table2 ON join_condition1INNER JOIN table3 ON join_condition2;

In this example, we have used a left outer join between “table1” and “table2” and an inner join between “table2” and “table3”.

Conclusion

Left outer join is a powerful tool that allows you to combine data from two or more tables, even if there are no matching rows between them. By using left outer join in your SQL Server queries, you can retrieve all the data you need for your analysis and reporting purposes. We hope this article has helped you understand left outer join and how to use it effectively. Happy coding!