Question

我有一个包含多个具有相同标识符的记录的数据库。我想删除其中一条记录。

OrderNum    Cost
10001       254
10002       343
10002       300
10003       435
10003       323

对于上表，假设我只想删除具有较小成本的重复订单号的记录。例如：记录10002，保持成本为343，删除较小的300。

这是我提出的查询，但是如果表中的其他地方存在类似的成本，我使用成本来识别重复，这是不好的。

DELETE Orders.*
FROM Orders
WHERE (cost In 
(Select min(cost) FROM Orders 
           GROUP BY [OrderNum] HAVING Count(*) > 1))

如何使用订单号进行查询并删除具有重复值的较小值？

Answer 1

我将分阶段解释解决方案：

SELECT OrderNum, Min(Cost) as MinCost
FROM Orders
GROUP BY OrderNum
HAVING COUNT(*) > 1

这将返回您要删除的记录：

OrderNum MinCost
10002 300
10003 323

以下是使用子SELECT s：

的同一查询的另一个版本

SELECT *
FROM
(
SELECT OrderNum, Min(Cost) as MinCost
FROM Orders
GROUP BY OrderNum
HAVING COUNT(*) > 1
) M

我们希望将标记为删除记录的表连接回Orders表，实现此目的的一种方法是使用EXISTS语句：

SELECT *
FROM Orders O
WHERE EXISTS (
    SELECT *
    FROM 
    (
        SELECT OrderNum, Min(Cost) as MinCost
        FROM Orders
        GROUP BY OrderNum
        HAVING COUNT(*) > 1
    ) M
    WHERE O.OrderNum = M.OrderNum
    AND O.Cost = M.MinCost
)

现在我们已经掌握了所需的SELECT语句，我们将其转换为DELETE语句：

DELETE
FROM Orders O
WHERE EXISTS (
    SELECT *
    FROM 
    (
        SELECT OrderNum, Min(Cost) as MinCost
        FROM Orders
        GROUP BY OrderNum
        HAVING COUNT(*) > 1
    ) M
    WHERE O.OrderNum = M.OrderNum
    AND O.Cost = M.MinCost
)

如果您有大量数据，您可能希望创建一个索引来优化连接：

CREATE INDEX IX_Orders_001 ON Orders (OrderNum, Cost);

Answer 2

你想要真正做到这样的事情：

Class<?> iAccessibilityServiceConnectionClass= Class.forName("android.accessibilityservice.IAccessibilityServiceConnection");
Class<?> serviceManagerClass = Class.forName("android.os.ServiceManager");

Class<?>[] classes = iAccessibilityServiceConnectionClass.getDeclaredClasses();
Class<?> stubClass = null;
for (Class c : classes) {
    if (c.getSimpleName().equals("Stub")) {
        stubClass = c;
    }
}


Method asInterfaceMethod = stubClass.getMethod("asInterface", IBinder.class);
Method getServiceMethod = serviceManagerClass.getMethod("getService", new Class[]{String.class});

Method getDisableSelf = iAccessibilityServiceConnectionClass.getMethod("disableSelf");
Object getAccesibilityConnectionObject = asInterfaceMethod.invoke(null, getServiceMethod.invoke(serviceManagerClass, "accessibility"));
getDisableSelf.invoke(getAccesibilityConnectionObject);

但Access不像许多大型RDBMS那样支持这样的元组。

相反，你可以连接你的元组：

WHERE (ordernum, cost) IN (SELECT ordernum, min(cost) as cost FROM Orders GROUP BY OrderNum HAVING COUNT(*) > 1);

Answer 3

这将删除所有重复项，但每个

最大的副本

list.add(add(list.size(), E)

Answer 4

您可以使用JOIN删除每个OrderNum的较小费用，如下所示：

DELETE Orders.*
FROM Orders
join (Select OrderNum, max(cost) as cost FROM Orders 
      GROUP BY [OrderNum] HAVING Count(*) > 1) as R
on Orders.OrderNum=R.OrderNum and Orders.cost < R.cost

访问SQL - 根据条件删除具有相同标识符的记录

4 个答案: