There comes a time when answering a question with data that not all the information needed is within a singular table. That is where knowing how to join tables becomes invaluable.

We will go through an understanding of Joins and how to apply them in MySQL.

What are Joins?

Joins help you connect multiple tables that have a column in common. The tables below have a relationship, highlighted in red, that we can use to connect the tables.

Employees

Offices

Types of Joins

• LEFT JOIN
• RIGHT JOIN
• INNER JOIN
  • SELF-JOIN

Left Join

The returned table shows all rows from the left table (employees) and rows that have a matching value with the right table (offices).

SELECT * 
FROM employees
LEFT JOIN offices
ON employee.office_code = offices.office_ID

Right Join

The returned table shows all rows from the right table (offices) and rows that have a matching value with the left table (employees).

SELECT * 
FROM employees
RIGHT JOIN offices
ON employees.office_code = offices.office_ID

As you can see above, the second table (offices) returned the Busan office details, although there are no recorded employees at this location.

Inner Join

The table will only return rows with the same value in common from both tables.

SELECT * 
FROM employees
INNER JOIN offices
ON employees.office_code = offices.office_ID

Self-Join

Duplicates a singular table, treating each version as unique and joining them into one. But why would you need a table to do this? In the case of hierarchical data.

Each row in the Employees’ table references their boss’s ID number. We may want the information of the higher-up to show on the same row as the subordinate.

-- I'm narrowing down the columns viewed after the join
SELECT emp.ID, emp.first_name, emp.last_name, emp.office_code, 
       emp.boss_ID, boss.first_name AS boss_fname, 
       boss.last_name as boss_lname, boss.office_code AS boss_office_code
FROM employees AS emp
INNER JOIN employees AS boss
ON emp.boss_ID= office_table.ID