Self JOIN

SELF JOIN

The SELF JOIN keyword joins a table with itself to compare rows within the same table.

💡 Joins a table with itself to compare rows within the same table

Visual Diagram

Table A       JOIN        Table B
┌─────────┐             ┌─────────┐
│  Rows   │  ────────→  │  Rows   │
│  from   │  matched    │  from   │
│  A      │  by key     │  B      │
└─────────┘             └─────────┘

Basic Syntax

SELECT columns
FROM table1
SELF JOIN table2
ON table1.column = table2.column;

Example

SELECT
    c.first_name,
    c.last_name,
    o.total AS order_total
FROM customers c
SELF JOIN orders o ON c.id = o.customer_id;

⚠️ Always specify the join condition in the ON clause. Omitting it creates a CROSS JOIN (Cartesian product)!

✅ Key Takeaways

1. SELF JOIN combines rows from two tables based on a related column
2. The ON clause specifies how the tables are related
3. Use table aliases to make queries more readable
4. Different join types determine which rows are included
5. Practice joins with sample data to understand the differences