如何編寫查詢以在分組行中查找重複值？

Question

表：

locations 
--------- 
contract_id INT 
order_position INT # I want to find any bad data in this column 

我想給定contract_id內找到任何重複order_position秒。

因此，例如，像如果兩個locations行過的「24845」一個contract_id兩者具有「3」的order_position任何情況。

我正在使用MySQL 5.1。

Answer 1

HAVING子句可以告訴你在哪裏重複存在的實例：

SELECT * 
FROM locations 
GROUP BY contract_id, order_position 
HAVING COUNT(*) > 1 

Answer 2

select contract_id, order_position 
from locations 
group by contract_id, order_position 
having count(*) > 1 

Answer 3

SELECT contract_id, order_position, count(*) 
FROM locations 
GROUP BY contract_id, order_position 
HAVING count(*) > 1 

Answer 4

SELECT contract_id, order_position, COUNT(order_position) AS cnt 
FROM locations 
GROUP BY contract_id, order_position 
HAVING cnt > 1; 

Why COUNT(column) is usually better than COUNT(*)（好吧，這只是區分我的答案：d）