# LeetCode 1069: Product Sales Analysis II

## Problem Restatement

Using the `Sales(sale_id, product_id, year, quantity, price)` table, report the total quantity sold for each product.

## Input and Output

| Table | Schema |
|---|---|
| `Sales` | `sale_id INT`, `product_id INT`, `year INT`, `quantity INT`, `price INT` |

Return a table with `product_id` and `total_quantity`.

## Edge Cases

- Match the aggregate to the question: use raw `COUNT` or `SUM` for rows/amounts, and `COUNT(DISTINCT ...)` only for unique entities.
- Tie cases should return every qualifying row unless the statement asks for a single row.
- For outer joins, keep filters on the joined table in the `ON` clause when unmatched rows must remain visible.

## Common Pitfalls

- Do not put aggregate filters in `WHERE`; use `HAVING` after `GROUP BY`.
- When the answer needs all tied winners, avoid `LIMIT 1` unless the statement explicitly asks for one row.

## Implementation

```sql
SELECT product_id, SUM(quantity) AS total_quantity
FROM Sales
GROUP BY product_id;
```

## Code Explanation

`GROUP BY product_id` groups all rows for the same product.

`SUM(quantity)` aggregates the total quantity sold for each group.