Self Join is a technology in MySQL that connects the same table to itself through alias. It is often used to process hierarchical or parent-child relationship data. For example, in the employees table, use LEFT JOIN to associate employees with their boss information: SELECT e.name AS employee_name, m.name AS manager_name FROM employees e LEFT JOIN employees m ON e.manager_id = m.id; this query can obtain the name of each employee and his direct boss, which is suitable for scenarios such as organizational structure, recursive data, etc. You need to pay attention to using alias, avoid circular references, and optimize performance.
In MySQL queries, self join is a technique of joining a table with itself. This query method is often used to process data with hierarchical structures or parent-child relationships, such as the relationship between employees and bosses, the nested structure of classification, etc.
Below is a specific self join example to help you understand its usage scenarios and writing.
What is Self Join?
Self Join is not a special JOIN type, but refers to the same table being associated with itself through alias . It is usually implemented using INNER JOIN or LEFT JOIN.
For example: Suppose there is a employees table, where each employee has a manager_id field pointing to id of his direct boss. If we want to find out the names of each employee and his boss, we can use self join.
How to write a Self Join query?
Let's look at this table structure:
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(50),
manager_id INT
);The table data is as follows:
| id | name | manager_id |
|---|---|---|
| 1 | Alice | NULL |
| 2 | Bob | 1 |
| 3 | Charlie | 1 |
| 4 | David | 2 |
Now we want to find out each person's name and their boss' name (if it exists), and you can write it like this:
SELECT e.name AS employee_name, m.name AS manager_name FROM employees e LEFT JOIN employees m ON e.manager_id = m.id;
Output result:
| Employee_name | manager_name |
|---|---|
| Alice | NULL |
| Bob | Alice |
| Charlie | Alice |
| David | Bob |
In this example:
- Two aliases were given to the
employeestable:erepresents the employee himself, andmrepresents the corresponding boss. -
LEFT JOINis used to keep records without bosses (such as Alice).
Common uses of Self Join
1. Query hierarchical relationships
For example, in scenarios such as organizational structure, comment reply chain, directory tree, etc., you can use self join to connect multiple times to find a parent-child relationship.
2. Compare different rows in the same table
For example, find customers with continuous sales growth, or compare the previous and last login times of the same user.
3. Build a recursive data structure
Although MySQL does not support recursive queries (CTE) until newer versions, multiple self-joins can be used to implement multi-layer relationship queries in some simple scenarios.
Notes and tips
- Alias ??is a must : because you want to use one table as two, you must give different alias to the table.
- Avoid dead loops : If there are circular references in the data (such as A manages B, B manages A and A), it may cause the query to fall into a dead loop.
- Performance issues : Self join may affect performance under large data volumes, especially when multiple nested connections, pay attention to index optimization.
Basically that's it. Although Self join seems a bit confusing, once you master its logic and usage scenarios, you can solve many practical problems.
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