Left Join vs Right Join

In SQL, JOINS represent the most basic tool for joining rows from two or more tables across a related column. Among

l="noreferrer noopener">SQL joins, LEFT JOIN and RIGHT JOIN are very frequently used to retrieve data. While both joins help merge data from more than one table, they differ in how to handle unmatched rows. We will here discuss SQL LEFT JOIN vs RIGHT JOIN, syntax, and when to use each along with examples and tables for further understanding.

Understanding SQL Joins

SQL JOIN in Relational Databases SQL joins in relational databases link data from two tables based on a condition. Fundamentally, it shares records between two tables by using columns that share a relationship. The types of joins in SQL include:

• INNER JOIN: Returns only matching rows between two tables.
• LEFT JOIN (or LEFT OUTER JOIN): Returns the rows of the left table and the rows of the right table for which the join condition is met. If no rows match, NULL values are returned on the columns of the right table.
• RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows of the right table and the rows of the left table where data is found in both. Returns NULL on the left columns if no match is available in the left table.

Left Join SQL: Explained

SQL’s LEFT JOIN returns all records from the left table, also known as the first table in the SQL statement, along with the matched records from the right table, also known as the second table. NULL values are returned for the columns of the right table whenever there is no match. That way, it is useful for queries in which you require all records of a left table to be included in the result, though they may not have a corresponding entry in the right table.

LEFT JOIN Syntax

The basic syntax for LEFT JOIN is as follows:

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column = table2.column;

Example of LEFT JOIN SQL

Let’s consider two tables: Employees and Departments.

Employees Table

EmployeeID	Name	DepartmentID
1	Alice	101
2	Bob	102
3	Charlie	NULL
4	David	103

Departments Table

DepartmentID	DepartmentName
101	Sales
102	Marketing
104	IT

If we use a LEFT JOIN to combine these tables, the result will include all employees, even if they don’t have a matching department:

SELECT Employees.Name, Departments.DepartmentName
FROM Employees
LEFT JOIN Departments
ON Employees.DepartmentID = Departments.DepartmentID;

Result of LEFT JOIN

Name	DepartmentName
Alice	Sales
Bob	Marketing
Charlie	NULL
David	NULL

Explanation:

• Alice and Bob have matching departments in the Departments table.
• Charlie and David do not have a match, so NULL is returned for their department.

Right Join SQL: Explained

It is pretty much identical to LEFT JOIN, except that it retrieves all rows from the right table with left rows that match up. NULL values will be returned for columns from the left table if there is no match.

RIGHT JOIN Syntax

The basic syntax for RIGHT JOIN is:

SELECT columns
FROM table1
RIGHT JOIN table2
ON table1.column = table2.column;

Example of RIGHT JOIN SQL

Using the same Employees and Departments tables, we can perform a RIGHT JOIN:

SELECT Employees.Name, Departments.DepartmentName
FROM Employees
RIGHT JOIN Departments
ON Employees.DepartmentID = Departments.DepartmentID;

Result of RIGHT JOIN

Name	DepartmentName
Alice	Sales
Bob	Marketing
NULL	IT

Explanation:

• Alice and Bob have matching departments.
• Since no employee belongs to the IT department (ID 104), NULL is returned for the employee’s name.

SQL LEFT JOIN vs RIGHT JOIN

Both LEFT JOIN and RIGHT JOIN join two tables and have included all rows where there is no match in one of the tables. The key difference is just in how they give priority to the tables:

• LEFT JOIN includes all rows from the left table (and NULLs for non-matches from the right table).
• RIGHT JOIN includes all rows from the right table (and NULLs for non-matches from the left table).

Difference Between Left and Right Join

Feature	LEFT JOIN	RIGHT JOIN
Priority Table	Returns all rows from the left table.	Returns all rows from the right table.
Non-Matching Rows	Non-matching rows from the right table are returned as NULL.	Non-matching rows from the left table are returned as NULL.
Usage	Used when you want to keep all rows from the left table, regardless of matches.	Used when you want to keep all rows from the right table, regardless of matches.

SQL JOIN Examples: LEFT JOIN vs RIGHT JOIN

Let’s use a more complex example to further illustrate the difference.

Orders Table

OrderID	CustomerID
1	101
2	102
3	103

Customers Table

CustomerID	CustomerName
101	John
104	Lisa

LEFT JOIN Example

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
LEFT JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

Result of LEFT JOIN

OrderID	CustomerName
1	John
2	NULL
3	NULL

RIGHT JOIN Example

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
RIGHT JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

Result of RIGHT JOIN

OrderID	CustomerName
1	John
NULL	Lisa

Explanation:

• LEFT JOIN includes all orders, even if there’s no matching customer. Hence, orders 2 and 3 have NULL for the customer name.
• RIGHT JOIN includes all customers, even if they haven’t placed an order. Hence, Lisa appears, but with NULL for the order.

When to Use LEFT JOIN or RIGHT JOIN

• Use LEFT JOIN if you want to get all of your records from the left table, even if there are no matching records in the right table.
• Use RIGHT JOIN when you want all the records from the right table, even when there are no matching records in the left table.

Some examples of when to use each are as follows:

LEFT JOIN Use Case:
Suppose you have a list of products and sales, but you wish to see all the products, whether sold or not.

SELECT Products.ProductName, Sales.SaleDate
FROM Products
LEFT JOIN Sales
ON Products.ProductID = Sales.ProductID;

RIGHT JOIN Use Case:

Suppose that you have a list of users and a list of the actions performed by them. You would like to get all actions, including those of users who have not taken any action so far.

SELECT Users.UserName, Actions.ActionType
FROM Users
RIGHT JOIN Actions
ON Users.UserID = Actions.UserID;

LEFT JOIN vs RIGHT JOIN: Performance Considerations

In general, LEFT JOIN and RIGHT JOIN are functionally identical. However, unless you specify otherwise, the LEFT JOIN variant is used in most JOIN operations. Which one to use depends on what table you want to focus on. For very big datasets, the performance may differ between LEFT JOIN and RIGHT JOIN. The reason is that different database engines implement these two types of joins slightly differently. Remember to also place indexes on columns referenced by join clauses for better performance.

Advantages of Left Join vs Right Join

Both SQL LEFT JOIN and RIGHT JOIN are used to combine rows from two or more tables based on a related column. However, they differ in how they handle unmatched rows from the two tables. Here’s a comparison of the advantages of using LEFT JOIN over RIGHT JOIN and vice versa:

Advantages of LEFT JOIN Over RIGHT JOIN

1. More Common and Intuitive to Use

• Widely Used in Practice: LEFT JOIN is more commonly used and generally easier to understand. Most SQL queries naturally retrieve data from a “primary” or “left” table and bring in related data from a secondary or “right” table. This convention makes LEFT JOIN more intuitive and widely understood by developers.

2. Preserves Data from the Primary Table

• Ensures Inclusion of All Rows from the Left Table: A LEFT JOIN guarantees that all records from the left table are returned, even if there is no match in the right table. This is beneficial when you want to ensure that no data from the left table is excluded, and you’re adding information from the right table where available.

3. Fewer Logical Reversals

• Avoids Rewriting Queries: Using a LEFT JOIN avoids the need to reverse the roles of the tables. In cases where the left table is the primary table (more significant in your query context), a LEFT JOIN keeps the query logic straightforward, without needing to switch the primary and secondary tables.

4. Simplifies Report Generation

• Left-Table-Focused Queries: In many reporting and data retrieval scenarios, the left table contains the main entities (e.g., customers, orders), and the right table contains supplementary data (e.g., payments, shipment details). LEFT JOIN is better suited for such cases because it ensures that all left-table data is represented in the final output.

5. Easier for Data-Enrichment Scenarios

• Main Data with Optional Details: LEFT JOIN is useful in scenarios where you want to enrich your main dataset (left table) with optional data (right table) without losing any records from the main dataset. For instance, retrieving customer information (left table) along with optional purchase history (right table) is more naturally achieved with a LEFT JOIN.

Advantages of RIGHT JOIN Over LEFT JOIN

1. Useful for Special Cases Where the Right Table is Primary

• Right Table Focus: If the right table holds the more critical or dominant data in a query, RIGHT JOIN can be more appropriate. For example, if you need to ensure that all rows from the right table are included, even if there’s no match in the left table, RIGHT JOIN can be the better choice.

2. When Data is Structured in Reverse Order

• Right-Side Priority in Queries: In some database designs or queries, the logical primary data might reside in the right table, and using RIGHT JOIN can prevent unnecessary table flipping or restructuring. It’s particularly useful when your data relationships are oriented more toward the right table.

General Comparison Advantages

1. Both Treat Unmatched Rows

• NULLs for Missing Matches: Both LEFT JOIN and RIGHT JOIN deal with unmatched rows by returning NULL for columns in the secondary table (right table in LEFT JOIN, left table in RIGHT JOIN). They come in handy when you want to include records from one table even if there is no matching record in the other table.

2. Same Logically Equivalent Outputs with Table Swapping

• Similarly equivalent behavior with swapped tables: LEFT JOIN and RIGHT JOIN can almost always be gotten by swapping the tables. For example, a LEFT JOIN on table_A and table_B is logically equivalent to a RIGHT JOIN on table_B and table_A. That gives you an extra degree of freedom about how you will structure your queries.

Disadvantages of Left Join vs Right Join

SQL LEFT JOIN and RIGHT JOIN are two types of joining data from more than one table. The only difference is what will happen if the rows in either of the joined tables do not match. And here’s a comparison of disadvantages for using LEFT JOIN over RIGHT JOIN, including specific drawbacks for each.

Disadvantages of LEFT JOIN Over RIGHT JOIN

1. Unwanted Data May Be Present in the Output

Includes All Rows from the Left Table: LEFT JOIN Always includes all rows from the left table, even if there are no matches in the right table. This can result in superfluous, potentially irrelevant data in your results if you don’t need all the left-side records. You will then have to filter again to remove unwanted rows.

2. Can Lead to Performance Issues on Large Left Tables

Heavy Data Load from Left Table: LEFT JOIN fetches all the rows from the left table; very heavy data loads on the left side slow the query down considerably. It could be especially problematic in case of complicated join conditions or if the right table contains many more null-matching records.

3. Requires NULL value management

Handling NULLs on the right table columns: Because LEFT JOIN returns NULL for unmatched rows from the right table, you’ll want to be careful with how you handle nulls in your result set. Depending on the logic in your data, you will be adding more conditions or perhaps some default values, which only adds more complexity to your query.

4. Right-Table-Focused Queries Can Be Awkward

Less Intuitive for Right-Table Centered Queries: LEFT JOIN is not as apt for queries wherein the right table holds the more important or dominant data. In those queries where the primary data comes from the right table, the application of LEFT JOIN feels somewhat awkward or counterintuitive and might make a query more difficult to read and understand.

Disadvantages of RIGHT JOIN Over LEFT JOIN

1. Less Commonly Used and Understood

• Not as Intuitive: RIGHT JOIN is used much less frequently than LEFT JOIN, and many developers are less familiar with its usage. This can make SQL queries that use RIGHT JOIN harder for others to understand, maintain, or debug. In many cases, people may need to mentally reverse the logic, which can be confusing.

2. Potential Confusion with Table Ordering

• Requires Reversing Table Order in Query Logic: If you decide to switch from a LEFT JOIN to a RIGHT JOIN, you typically need to reverse the order of the tables in your query, which can be unintuitive and confusing. The reversal can lead to errors or misunderstandings about which table’s data is being preserved.

3. Risk of Missing Data from Left Table

• Losing Critical Left-Table Data: With RIGHT JOIN, all records from the right table are returned, but rows from the left table are included only when there’s a match. If the left table contains crucial data that doesn’t match any rows in the right table, those rows will be omitted, which may result in incomplete data in the query output.

4. Harder to Debug and Maintain in Practice

• Complexity in Query Maintenance: RIGHT JOIN can make queries harder to debug and maintain because most SQL code and examples use LEFT JOIN. This unfamiliarity with RIGHT JOIN can cause difficulties when developers need to understand or modify the query, leading to potential errors or confusion.

5. Redundant in Most Cases

• Rarely Necessary: In most cases, RIGHT JOIN is not necessary because a LEFT JOIN can achieve the same result by simply switching the order of the tables. As a result, RIGHT JOIN is often seen as redundant, which may lead to confusion and inefficiency in SQL query writing.

General Drawbacks of Both

1. Slow Query Execution When Using Large Data Sets

Both Joins Become Very Slow: Right Join and left Join will be very slow. Large tables and complex join conditions lead to slow execution especially with the absence of index or if optimized.

2. Nulls in Unmatched Rows

Null values must be treated in special ways: Both joins will have nulls for the unmatched rows, which requires special handling in the query logic. Using null might make it more laboring to write a query and require extra steps where one would be looking for unexpected results or error in calculations.

3. Not very intuitive even to complex queries

When there are more than one table, then the result might come out to be quite complex if using LEFT and RIGHT JOIN because they can easily generate a SQL query that is not readable or maintainable or even optimization. Especially when it becomes ambiguous about which join to use in complex scenarios.