A relational database consists of multiple related tables linked together by common attributes called relationships.

These relationships can be used to combine the data from multiple tables simultaneously in a single query through a mechanism called a join.

Setting up the database

Before diving into this lesson, you will have to perform some modifications on your database.

1. Add a new column named manager_id to the employees table:

   mysql> ALTER TABLE employees ADD COLUMN manager_id INT NULL;

2. Create a relationship between the id column and the manager_id column:

   mysql> ALTER TABLE employees ADD CONSTRAINT fk_manager FOREIGN KEY (manager_id) REFERENCES employees(id);

3. Update the records of the employees table:

   mysql> UPDATE employees SET manager_id = 9 WHERE id = 2;