DML_Data Modification_DELETE

DML_Data Modification_Delete
删除记录比较简单，但是需要特别注意，一不小心，就变成了 “从删库到跑路“ 就掉的大了

/*
    Microsoft SQL Server 2008 T-SQL Fundamentals_CN（Chapter 8 Inserting Data）
*/

----------------------------------------------------------------------------------------------------------------------
----先不用曾经测试过的表做实验，还需要实验+看数据+回顾，不小心已经破坏了Orders表
--SELECT *
--FROM Orders
----------------------------------------------------------------------------------------------------------------------
--先复制：Northwind.Customers,    Orders，2个表(表结构+数据)到testdb库下
if OBJECT_ID('dbo.Customers','U') IS NOT NULL DROP TABLE dbo.Customers;
if OBJECT_ID('dbo.Orders','U') IS NOT NULL DROP TABLE dbo.Orders;

SELECT * INTO testdb.dbo.Customers FROM Northwind.dbo.Customers
SELECT * INTO testdb.dbo.Orders FROM Northwind.dbo.Orders

ALTER TABLE dbo.Customers ADD
    CONSTRAINT PK_Customers PRIMARY KEY(CustomerID)

ALTER TABLE dbo.Orders ADD
   CONSTRAINT PK_Orders PRIMARY KEY(OrderID),
   CONSTRAINT PK_Orders_Customers FOREIGN KEY(CustomerID)
         REFERENCES dbo.Customers(CustomerID)
-------------------------------------------------------------------------------
--检查，表结构+数据，都复制到 testdb 库下了
SELECT * FROM Customers
SELECT * FROM Orders
--
--SELECT * FROM Northwind.dbo.Customers
SELECT * FROM Northwind.dbo.Orders
        left join Northwind.dbo.Customers
        on Orders.CustomerID=Customers.CustomerID
WHERE ShipCountry = 'UK'                             -- 有 56个，为下面删除 连接...的参考
--------------
SELECT * FROM Northwind.dbo.Orders
        left join Northwind.dbo.Customers
        on Orders.CustomerID=Customers.CustomerID
WHERE ShipCountry = 'USA'                            -- 有 122个，为下面删除 连接...的参考
--------------
SELECT * FROM Northwind.dbo.Orders
        left join Northwind.dbo.Customers
        on Orders.CustomerID=Customers.CustomerID
WHERE ShipCountry = 'France'                         -- 有 77个，为下面删除 连接...的参考

-------------------------------------------------------------------------------
--   DELETE（没有过滤条件时，是删除所有行）
SET NOCOUNT OFF          --NOCOUNT 为 ON 时，只会报告命令完成，不会报告数量
DELETE
FROM dbo.Orders
WHERE orderdate < ''

--TRUNCATE 没有过滤条件，删除所有行
--区别：  TRUNCATE以最小模式记录日志(效率高)，DELETE以完整模式记录日志
--创建一个 dummy table（让它包含一个指向产品表的外键，这样就可保护产品表了，此处还可多理解一下
TRUNCATE TABLE Orders
-------------------------------------------------------------------------------
-- 基于连接的 DELETE 不是标注的SQL语句（知道有这么一种写法就可），平时尽量采用标准的SQL语句
DELETE FROM O
FROM dbo.Orders as O
    JOIN dbo.Customers AS C
    ON O.CustomerID = C.CustomerID
WHERE C.Country = 'UK'

--删除完满足条件的纪录后，再查，确证删除
--SELECT * FROM Customers
SELECT * FROM Orders
-----------------------------
--下面是标准的SQL语句
DELETE FROM dbo.Orders
WHERE EXISTS
(SELECT *
 FROM dbo.Customers AS C
 WHERE Orders.CustomerID = C.CustomerID
      AND Orders.ShipCountry ='USA'
)
--------
--以连接形式的写法，然后再删除
DELETE FROM dbo.Orders
WHERE EXISTS
(SELECT *
 FROM dbo.Customers AS C
 LEFT JOIN Orders AS O
 ON O.CustomerID = C.CustomerID
  WHERE  Orders.ShipCountry ='France'
)

--对于非标准的删除写法，也好理解(如右边)，   先查询--->后条件--->最后删除

运行结果：

略

/*
    Orders表在PIVOT、UNPIVOT中已经做了示例，下面表为OrdersA为区别
    Microsoft SQL Server 2008 T-SQL Fundamentals_CN（Chapter 8 Inserting Data）
*/