mysql - 如何编写查询以在分组行中查找重复值？

Question

桌子：

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

我想order_position在给定的contract_id.

因此，例如任何情况，例如如果两locations行的 acontract_id为“24845”，并且两者的 aorder_position为“3”。

我正在使用 MySQL 5.1。

Answer 1

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

Answer 2

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

Answer 3

HAVING 子句可以显示存在重复项的实例：

SELECT *
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;

为什么COUNT(column)通常比COUNT(*)（好吧，这只是为了区分我的答案：D）