List Email Spellings That Occur More Than Once
Company: Google
Role: Software Engineer
Category: Data Manipulation (SQL/Python)
Difficulty: easy
Interview Round: Other
### Problem
List every duplicate email from Person once, under the output label Email. The email field is guaranteed non-NULL and contains no uppercase letters. Row ordering is unrestricted.
### Tables
Table: `Person`
```text
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| id | int |
| email | varchar |
+-------------+---------+
id is the primary key (column with unique values) for this table.
Each row of this table contains an email. The emails will not contain uppercase letters.
```
### Example
```text
Person (id,email): (1,a@example.net), (2,b@example.net), (3,a@example.net), (4,a@example.net)
Output (Email): (a@example.net)
```
### What a Strong Answer Covers
- Report each duplicated spelling exactly once.
- Keep the requested output label and leave single occurrences out.
### Follow-up Questions
- How would you report the number of occurrences alongside each duplicate?
- How would you change the rule if email matching were case-insensitive?
Overview: Return each email spelling that appears more than once, with one row for each repeated spelling.