When or why would you use a right outer join instead of left with example?

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A right outer join, also known as a right join or a right outer join, is a type of join in which all the rows from the right table are included, along with matching rows from the left table. Any rows in the right table that do not have a matching row in the left table will contain null values for the left table’s columns.

Query:

Here’s an example query for a right outer join:

SELECT *FROM Table1RIGHT JOIN Table2ON Table1.key = Table2.key;

In this query, we’re performing a right outer join between Table1 and Table2, based on a matching key column.

Example:

Let’s consider an example with multiple tables. Suppose we have the following three tables:

  • employees (emp_id, emp_name, dept_id)
  • departments (dept_id, dept_name)
  • salaries (emp_id, salary)
employees+--------+----------+---------+| emp_id | emp_name | dept_id |+--------+----------+---------+| 1      | Alice    | 1       || 2      | Bob      | 2       || 3      | Charlie  | 1       || 4      | David    | NULL    |+--------+----------+---------+departments+---------+------------+| dept_id | dept_name  |+---------+------------+| 1       | Sales      || 2       | Marketing || 3       | IT         |+---------+------------+salaries+--------+--------+| emp_id | salary |+--------+--------+| 1      | 50000  || 2      | 60000  || 3      | 55000  || 5      | 45000  |+--------+--------+

Suppose we want to get a list of all employees and their corresponding department name and salary, including those employees who do not have a corresponding department or salary.

To accomplish this, we can use a right outer join between the employees and departments tables, and a left outer join between the employees and salaries tables, as follows:

SELECT employees.emp_id, employees.emp_name, departments.dept_name, salaries.salaryFROM employeesRIGHT JOIN departments ON employees.dept_id = departments.dept_idLEFT JOIN salaries ON employees.emp_id = salaries.emp_id;

In this query, we first perform a right outer join between the employees and departments tables to include all the employees and their department name (if available). We then perform a left outer join between the employees and salaries tables to include the salary for each employee (if available).

The output of this query would be:

+--------+----------+------------+--------+| emp_id | emp_name | dept_name  | salary |+--------+----------+------------+--------+| 1      | Alice    | Sales      | 50000  || 2      | Bob      | Marketing | 60000  || 3      | Charlie  | Sales      | 55000  || 4      | David    | NULL       | NULL   |+--------+----------+------------+--------+

Note that all employees are included in the result set, along with their corresponding department name (if available) and salary (if available). However, since there is no matching salary for the employee with emp_id=4, the salary column contains a null value for that row.

Conclusion

A right outer join is a type of outer join that returns all the rows from the right table and the matching rows from the left table. In other words, a right outer join ensures that all the rows from the right table are included in the result set, along with the matching rows from the left table, if any. This can be useful in scenarios where we want to include all the records from a certain table, even if there is no matching record in the other table.

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