MySQL - Left Join Queries

A left join, also known as a left outer join, is another type of join in SQL. A left join returns 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 will contain null values for the right table columns.

To perform a left join in MySQL, you need to specify the tables to join and the columns to use for the join condition using the "ON" clause. You also need to use the "LEFT JOIN" keyword to specify the type of join.

Let's say you have two tables in your database: "employees" and "departments". The "employees" table contains information about employees, including their name, department ID, and salary. The "departments" table contains information about the various departments in your organization, including their name and ID.

You want to create a report that lists all departments and the total salary of all employees in each department. However, you also want to include departments that do not have any employees. To do this, you need to use a left join to include all departments in your result set.

Here's the SQL code for the left join:

SELECT departments.department_name, COALESCE(SUM(employees.salary), 0) as total_salary

FROM departments

LEFT JOIN employees

ON departments.department_id = employees.department_id

GROUP BY departments.department_name;

In this example, we're selecting the "department_name" column from the "departments" table and using the COALESCE function to handle any null values that might occur in the "salary" column of the "employees" table. We're also using the SUM function to calculate the total salary of all employees in each department.

We're using a left join to ensure that all departments are included in the result set, even if there are no matching employees in the "employees" table. If there are no matching employees, the result will contain null values for the "salary" column.

The result of this query will be a table that lists each department and the total salary of all employees in that department. If there are any departments that do not have any employees, they will still appear in the result set with a total salary of 0.

Another example of a left join in MySQL.

Let's say you have two tables in your database: "customers" and "orders". The "customers" table contains information about each customer, including the customer ID and the customer name. The "orders" table contains information about each order, including the order ID, the customer ID, and the order date.

You want to create a report that shows the total number of orders for each customer, including customers who have not placed any orders. To do this, you need to use a left join to include all customers in your result set, even if they have not placed any orders.

Here's the SQL code for the left join:

SELECT customers.customer_name, COUNT(orders.order_id) as total_orders

FROM customers

LEFT JOIN orders

ON customers.customer_id = orders.customer_id

GROUP BY customers.customer_name;

In this example, we're selecting the "customer_name" column from the "customers" table and using the COUNT function to calculate the total number of orders for each customer. We're using a left join to ensure that all customers are included in the result set, even if they have not placed any orders.

The result of this query will be a table that lists each customer and the total number of orders they have placed. If there are any customers who have not placed any orders, they will still appear in the result set with a total order count of 0.