# Joins

This page covers the argument forms of `innerJoin()`, `leftJoin()`, and `rightJoin()` and the SQL they produce.

## Three Joins

All three methods share one internal `join()` implementation (`src/MySQLPool.ts:151-174`) and differ only in the join type:

| Method | SQL |
|---|---|
| `innerJoin(table, first, operator, second?)` | `INNER JOIN` |
| `leftJoin(table, first, operator, second?)` | `LEFT JOIN` |
| `rightJoin(table, first, operator, second?)` | `RIGHT JOIN` |

## Argument Forms

| Form | Example | Generated SQL |
|---|---|---|
| Three arguments (equality) | `innerJoin("users", "orders.user_id", "users.id")` | `` INNER JOIN `users` ON orders.user_id = users.id `` |
| Four arguments (explicit operator) | `leftJoin("p", "a", "!=", "b")` | `` LEFT JOIN `p` ON `a` != `b` `` |

With three arguments, the third becomes `second` and the operator is `=`.

```typescript
const rows = await MySQLPool
  .table("orders")
  .select("orders.id", "users.name", "products.title")
  .innerJoin("users", "orders.user_id", "users.id")
  .leftJoin("order_items", "orders.id", "order_items.order_id")
  .leftJoin("products", "order_items.product_id", "products.id")
  .where("orders.status", "completed")
  .get();
```

## Table and Column Name Rules

| Target | Rule |
|---|---|
| Joined table | Always backtick-quoted |
| `first`, `second` | Backtick-quoted without `.`; passed through with `.` |

Column names often collide across tables, so use the `table.column` form in both `select()` and `where()` to avoid ambiguous column errors.

## Related Pages

- [Select and Where](/select-and-where)
- [Sorting and Pagination](/sorting-and-pagination)