SELF JOIN - (MySQL Join)

A self-join is a regular join, but the table is joined with itself. It is useful when you have a table that contains hierarchical data or when you need to compare records within the same table. In this tutorial, we'll discuss the syntax, example, output, explanation, use, important points, and summary of a self-join in MySQL.

Syntax

The syntax for a self-join in MySQL is as follows:

SELECT a.column1, b.column2
FROM table a
JOIN table b ON a.join_column = b.join_column

In the above syntax, "table" refers to the table being joined with itself, and "join_column" refers to the column being used for joining.

Example

Let's say we have a table called "employees" that contains information about employees and their managers. The table has two columns - "employee_name" and "manager_name":

employee_name	manager_name
Alice	John
Bob	John
Charlie	Bob
David	Charlie

We can use a self-join to retrieve a list of employees and their managers:

SELECT a.employee_name, b.employee_name AS manager_name
FROM employees a
JOIN employees b ON a.manager_name = b.employee_name

The above query uses a self-join to join the "employees" table with itself on the "manager_name" column.

Output

When we run the above query, we get the following output:

employee_name	manager_name
Alice	John
Bob	John
Charlie	Bob
David	Charlie

Explanation

In the above example, we used a self-join to join the "employees" table with itself on the "manager_name" column. We used aliases "a" and "b" to differentiate between the two instances of the "employees" table. We selected the "employee_name" column from the "a" instance of the table and the "employee_name" column again, this time with the alias "manager_name", from the "b" instance of the table.

Use

A self-join can be used to retrieve information about hierarchical data or to compare records within the same table. It is especially useful when dealing with organizational charts or family trees.

Important Points

• In a self-join, the table is joined with itself.
• Aliases should be used to differentiate between the two instances of the table.
• Self-joins can be useful for retrieving information about hierarchical data or for comparing records within the same table.

Summary

In this tutorial, we discussed the syntax, example, output, explanation, use, and important points of a self-join in MySQL. Self-joins are useful when you need to compare records within the same table or when you have a table that contains hierarchical data. With the knowledge of self-joins, you can now write efficient queries that retrieve data from the same table.