# LeetCode 1050: Actors and Directors Who Cooperated At Least Three Times

## Problem Restatement

We have a table `ActorDirector(actor_id, director_id, timestamp)`.

We need to return all `(actor_id, director_id)` pairs where the actor has cooperated with the director at least three times.

## Input and Output

| Table | Schema |
|---|---|
| `ActorDirector` | `actor_id INT`, `director_id INT`, `timestamp INT` |

Return a table with `actor_id` and `director_id`.

## Examples

```
ActorDirector:
+----------+-------------+----------+
| actor_id | director_id | timestamp |
+----------+-------------+----------+
| 1        | 1           | 0        |
| 1        | 1           | 1        |
| 1        | 1           | 2        |
| 1        | 2           | 3        |
| 1        | 2           | 4        |
| 2        | 1           | 5        |
+----------+-------------+----------+
```

Actor `1` worked with director `1` three times.

Answer:

```
+----------+-------------+
| actor_id | director_id |
+----------+-------------+
| 1        | 1           |
+----------+-------------+
```

## Key Insight

Group by the `(actor_id, director_id)` pair and count rows. Filter where count ≥ 3.

## 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 actor_id, director_id
FROM ActorDirector
GROUP BY actor_id, director_id
HAVING COUNT(*) >= 3;
```

## Code Explanation

`GROUP BY actor_id, director_id` groups each unique pair.

`COUNT(*)` counts the number of collaborations.

`HAVING COUNT(*) >= 3` keeps only pairs with at least three collaborations.