SQL选择具有（部分）重复数据的行

Question

I have the following database schema:

Product ID | Component | ...

Product ID - a foreign key

Component - parts of the Product

For some Arcane reason a number of Records have the same Product ID & Component. Is there a SQL query that would return all the Product ID's & Components, that have multiple identical Components?

Eg given the following table

| Product ID | Component |
--------------------------
| 1          | c1000     |
| 1          | c1100     |
| 2          | c2000     |
| 2          | c2000     |
| 2          | c2200     |
| 3          | c3000     |

The SQL query should return:

| Product ID | Component |
--------------------------
| 2          | c2000     |

Answer 1

SELECT ProductId, Component, count(*) Duplicates
 from MyTable  --  or whatever
 group by ProductId, Component
 having count(*) > 1

This will also show you how many duplicate entries there are.

Answer 2

SELECT
  ProductId,
  Component
FROM
  Table
GROUP BY
  ProductId,
  Component
HAVING
  COUNT(*) > 1

Answer 3

select "Product ID", Component
from table
group by "Product ID", Component
having count(*) > 1