Want to learn SQL and write efficient and better queries? Then it is essential to learn SQL JOINS to Write Better Queries. Let’s understand what SQL JOINS are all about and learn SQL JOINS with examples in the article below

JOINS are used in SQL to combine rows from two or more tables based on a related column between them. For example, if you have a “customers” table and a “orders” table and you want to retrieve all customer information along with their order details, you can use a JOIN to combine the two tables and get all the relevant data in a single result.

Types of JOINS in SQL with Examples

There are several types of joins in SQL –

• INNER JOIN: This type of join returns rows from both tables that satisfy the join condition.
• OUTER JOIN: This type of join returns all rows from both tables and fills in NULL values for missing matches on either side. There are three types of outer joins:
  • LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table, and any matching rows from the right table. If there is no match, NULL values are returned for right table’s columns.
  • RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows from the right table, and any matching rows from the left table. If there is no match, NULL values are returned for left table’s columns.
  • FULL OUTER JOIN: Returns all rows from both tables and fills in NULL values for missing matches on either side.
• CROSS JOIN: This type of join returns the Cartesian product of the two tables. In other words, it will return every possible combination of rows from the two tables.
• SELF JOIN: This type of join is used to join a table to itself. It is used to compare values in the same table, or to perform calculations using values in the same table.

An overview of all the above joins can be as seen below –

OUTER JOIN in SQL with Examples

An outer join returns all rows from both tables, and NULL values for non-matching rows.

LEFT JOIN in SQL with Examples

Here is an example of LEFT JOIN

SELECT *
FROM customers
LEFT JOIN orders
ON customers.customer_id = orders.customer_id;

This will return all rows from the customers table, and any matching rows from the orders table. Non-matching rows in the orders table will have NULL values for all columns.

RIGHT JOIN in SQL with Examples

Here is an example of RIGHT JOIN

SELECT *
FROM customers
RIGHT JOIN orders
ON customers.customer_id = orders.customer_id;

This will return all rows from the orders table, and any matching rows from the customers table. Non-matching rows in the customers table will have NULL values for all columns.

FULL OUTER JOIN in SQL with Examples

Here is an example of a FULL OUTER JOIN –

SELECT *
FROM customers
FULL OUTER JOIN orders
ON customers.customer_id = orders.customer_id;

This will return all rows from both the customers and orders tables, and NULL values for any non-matching rows.

INNER JOIN in SQL with Examples

An inner join returns rows that match the specified criteria in both tables.

Here is an example of an INNER JOIN –

SELECT *
FROM customers
INNER JOIN orders
ON customers.customer_id = orders.customer_id;

This will return only the rows where there is a matching customer_id in both the customers and orders tables.

Here is another example of INNER JOIN

SELECT customers.name, orders.order_date
FROM customers
INNER JOIN orders
ON customers.customer_id = orders.customer_id
WHERE orders.order_total > 100;

This will return the name of the customer and the order_date for any orders with an order_total greater than 100, but only for customers who have placed an order.

CROSS JOIN in SQL with Examples

A cross join returns the Cartesian product of the two tables, i.e., every row from the left table is paired with every row from the right table.

Here is an example of a cross join –

SELECT *
FROM customers
CROSS JOIN orders;

This will return every combination of rows from the customers and orders tables. If the customers table Here is another example:

SELECT customers.name, orders.product_name
FROM customers
CROSS JOIN orders
WHERE customers.state = 'California' AND orders.product_price > 100;

This will return the name of the customer and the product_name for every combination of rows in the customers and orders tables, but only for customers in California who have purchased a product with a price greater than 100.has N rows and the orders table has M rows, the resulting table will have N * M rows.

SELF JOIN in SQL with Examples

A SELF JOIN is a join that combines rows from a table with itself, typically for the purpose of comparing data within the same table.

Here is an example of a SELF JOIN –

SELECT t1.employee_id, t1.name AS employee_name, t2.name AS manager_name
FROM employees AS t1
INNER JOIN employees AS t2
ON t1.manager_id = t2.employee_id;

This will return the employee_id, employee_name, and manager_name for each employee, along with the name of their manager (if they have one). The employees table is aliased as t1 and t2 to distinguish the two instances of the table.

Here is another example:

SELECT t1.name AS employee_name, t2.name AS supervisor_name
FROM employees AS t1
LEFT JOIN employees AS t2
ON t1.supervisor_id = t2.employee_id;

This will return the employee_name and supervisor_name for each employee, along with the name of their supervisor (if they have one). If an employee does not have a supervisor, the supervisor_name will be NULL.

How to Choose a JOIN

When choosing a join type, you should consider the following factors:

• What type of relationship exists between the tables?
  • INNER JOIN: Tables have a matching relationship.
  • OUTER JOIN: Tables have a one-to-many relationship.
  • CROSS JOIN: Tables have no relationship.
  • SELF JOIN: Table has a many-to-many relationship with itself.
• What type of result set do you want to return?
  • INNER JOIN: Returns only matching rows.
  • OUTER JOIN: Returns all rows, with NULL values for non-matching rows.
  • CROSS JOIN: Returns every combination of rows from both tables.
  • SELF JOIN: Returns all rows from the table, paired with other rows from the same table.
• What type of join will give you the most efficient query?
  • INNER JOIN: Generally, the most efficient type of join.
  • OUTER JOIN: Can be less efficient than INNER JOIN, especially for large tables.
  • CROSS JOIN: Can be very inefficient for large tables.
  • SELF JOIN: Performance will depend on the size and complexity of the table.

In general, you should use inner joins unless you specifically need to include non-matching rows in the result set. If you are unsure which type of join to use, you can try both and compare the results to see which one produces the desired output.

Nested JOIN in SQL with Examples

A nested join, also known as a subquery join, is a type of join that uses a subquery to join two tables.

Here is an example of a nested join –

SELECT t1.*, t2.*
FROM customers AS t1
INNER JOIN (
SELECT *
FROM orders
WHERE order_total > 100
) AS t2
ON t1.customer_id = t2.customer_id;

This will return all customers who have placed an order with a total greater than 100, along with the details of those orders. The orders table is joined to the customers table using a subquery, which selects only the orders with a total greater than 100.

Nested joins can be used to perform more complex queries, and can be especially useful when working with large datasets. However, they can also be slower than other types of joins due to the additional processing required to evaluate the subquery.

Multiple JOINS in SQL with Examples

It is possible to use multiple joins in a single SQL statement, to combine data from multiple tables.

Here is an example of a query with multiple joins:

SELECT t1.name AS customer_name, t2.product_name, t3.order_date
FROM customers AS t1
INNER JOIN orders AS t2
ON t1.customer_id = t2.customer_id
INNER JOIN order_items AS t3
ON t2.order_id = t3.order_id;

This will return the customer_name, product_name, and order_date for each order, along with the name of the customer who placed the order. The customers table is joined to the orders table using an inner join, and the orders table is joined to the order_items table using another inner join.

When using multiple joins, it is important to make sure that the join conditions are correctly specified, to ensure that the correct data is returned. It is also a good idea to use table aliases (e.g., t1, t2, t3 in the example above) to help distinguish the different tables in the query.

Combining JOINS in SQL with Examples

It is possible to combine different types of joins in a single SQL statement. For example, you might use a LEFT JOIN to return all rows from one table, and a RIGHT JOIN to return all rows from another table.

Here is an example of a query that combines a LEFT JOIN and a RIGHT JOIN –

SELECT t1.*, t2.*
FROM customers AS t1
LEFT JOIN orders AS t2
ON t1.customer_id = t2.customer_id
RIGHT JOIN products AS t3
ON t2.product_id = t3.product_id;

This will return all rows from the customers and products tables, and any matching rows from the orders table. Non-matching rows in the orders table will have NULL values for all columns.

Using All JOINS in one query in SQL with Examples

Is it possible to use all the JOINS in a single query? The answer is a definite Yes!

Here is an example of a query that uses all of the different types of JOINs –

SELECT t1.*, t2.*, t3.*, t4.*
FROM customers AS t1
INNER JOIN orders AS t2
ON t1.customer_id = t2.customer_id
LEFT JOIN order_items AS t3
ON t2.order_id = t3.order_id
RIGHT JOIN products AS t4
ON t3.product_id = t4.product_id
FULL OUTER JOIN shipping_addresses AS t5
ON t1.customer_id = t5.customer_id
CROSS JOIN employees AS t6;

This will return all rows from the customers, orders, order_items, products, and shipping_addresses tables, as well as every combination of rows from the employees table. The customers table is joined to the orders table using an inner join, the orders table is joined to the order_items table using a left join, the order_items table is joined to the products table using a right join, the customers table is joined to the shipping_addresses table using a full outer join, and the employees table is joined to every other table using a cross join.

Advantages of JOINS

Some advantages of using JOINs include –

• Improved query performance – By retrieving data from multiple tables in a single query, you can often avoid the need for multiple separate queries, which can improve the performance of your application.
• Data consistency – JOINs can help ensure that related data is consistent across multiple tables. For example, you can use a JOIN to verify that a customer’s billing and shipping addresses are consistent.
• Simplified queries – JOINs can make it easier to write complex queries that would otherwise be difficult or impossible to express using a single table.

Disadvantages of JOINS

Some disadvantages of using JOINs include –

• Increased complexity – JOINs can make queries more complex, which can make them harder to understand and maintain.
• Reduced overall performance – JOINs can be slower than other types of queries, especially when working with large datasets.
• Increased risk of errors – JOINs can be prone to errors, such as incorrect join conditions or missing data, which can result in incorrect or incomplete results.

Conclusion to Learn SQL JOIN with examples

In conclusion, JOINs can be a useful tool for retrieving and combining data from multiple tables, but they should be used with care to ensure that they are used effectively and efficiently.

External Links

Some of the links that I believe will help further understand JOINs –

• W3 Schools
• EDUCBA
• Geek for Geeks
• Practice your Coding skills for free on HackerRank

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