When it comes to SQL, it’s important to understand the different types of joins that can occur between tables. Two of the most common join types are the left join and the left outer join. These two terms are often used interchangeably, but are they really the same thing?
Let’s take a closer look at each join type and explore their similarities and differences.
What is a Left Join?
A left join, also known as a left inner join, is a type of join where all rows from the left table are returned, along with any matching rows from the right table. If there is no matching row in the right table, the result will still include the row from the left table, but with null values for the right table columns.
To better understand a left join, let’s use an example. Consider two tables: “Customers” and “Orders”. The “Customers” table contains information about each customer, such as their name and email address. The “Orders” table contains information about each order, such as the order ID and order date.
To join these two tables using a left join, we would use the following syntax:
SELECT Customers.Name, Orders.OrderID
FROM Customers
LEFT JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;
This query would return all customers, regardless of whether or not they have placed an order. If a customer has placed an order, the query would also return the order ID.
What is a Left Outer Join?
A left outer join, also known as a left join or left outer join, is very similar to a left join in that all rows from the left table are returned. However, a left outer join also includes any non-matching rows from the right table.
To continue with our example, let’s modify the previous query to use a left outer join:
SELECT Customers.Name, Orders.OrderID
FROM Customers
LEFT OUTER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;
This query would return all customers, along with any orders they have placed. If a customer has not placed an order, the query would still return their information, but with null values for the order ID.
Are Left Join and Left Outer Join the Same Thing?
As you can see, a left join and a left outer join are very similar, but they do have one key difference: a left outer join includes non-matching rows from the right table.
So, why do some people use the terms left join and left outer join interchangeably? The answer likely lies in the fact that the outer keyword is optional in many SQL dialects. In fact, in some dialects, including MySQL and SQLite, the terms left join and left outer join are synonymous.
However, in other dialects, such as Oracle and SQL Server, the outer keyword is required to specify a left outer join. In these dialects, you would use the syntax “LEFT OUTER JOIN” to perform a left outer join, and “LEFT JOIN” to perform a left inner join.
Should You Use Left Join or Left Outer Join?
So, which type of join should you use? The answer depends on your specific use case.
If you only want to return matching rows between two tables, then a left join (or left inner join) is appropriate. This type of join is useful when you want to combine two tables but only care about the rows where there is a match in both tables.
If, on the other hand, you want to return all rows from one table, along with any matching rows from another table, then a left outer join is the right choice. This type of join is useful when you want to include all rows from one table, regardless of whether or not there is a match in another table.
Conclusion
In summary, a left join and a left outer join are similar in that they both return all rows from the left table. However, a left outer join includes non-matching rows from the right table, while a left join only includes matching rows.
The usage of the terms left join and left outer join may vary across different SQL dialects, but the important thing is to understand the difference between these join types and use them appropriately based on your specific use case.