LEFT JOIN

LEFT JOIN

The LEFT JOIN keyword returns all rows from the left table, plus matching rows from the right.

💡 Returns ALL rows from the LEFT table, plus matching rows from the RIGHT

Visual Diagram

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

Basic Syntax

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

Example

SELECT
    c.first_name,
    c.last_name,
    o.total AS order_total
FROM customers c
LEFT 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. LEFT 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