Question

假设我有两个表帐户和提款。帐户具有属性Balance。每次在表Withdraw中输入一个值时，我想检查是否有足够的余额这样做。如果是，我想从余额中减去该金额。

这不是我想要做的，而是我要求的简化版本。

PS：“检查约束”是正确的短语吗？我不太确定。谢谢！

Answer 1

这是一个简短的例子：

INSERT INTO mytable 
            (id, name) 
SELECT 1, 'test' 
WHERE  NOT EXISTS(SELECT id 
                  FROM   myanother_table 
                  WHERE  id = 1)

注意：如果不理解或不看，很难做出任何假设 OP的代码。所以我刚刚提供了一些例子。

Answer 2

您可以创建一个功能，以验证余额是否为＆gt;比你的@value 然后将该函数添加到约束中。

试着看看这个：

Can a Check constraint relate to another table?

Answer 3

你想要的不是约束。您正在寻找触发器。看一下这个例子。

IF EXISTS ( SELECT  *
            FROM    sys.objects
            WHERE   object_id = OBJECT_ID(N'[dbo].[Account]')
                    AND type IN ( N'U' ) )
    DROP TABLE [dbo].[Account]
GO

CREATE TABLE dbo.Account
    (
      AccountID INT NOT NULL ,
      AccountBalance DECIMAL(19, 2) NOT NULL
    )
GO

IF EXISTS ( SELECT  *
            FROM    sys.objects
            WHERE   object_id = OBJECT_ID(N'[dbo].[Transaction]')
                    AND type IN ( N'U' ) )
    DROP TABLE [dbo].[Transaction]
GO

CREATE TABLE dbo.[Transaction]
    (
      TransactionID INT NOT NULL
                        IDENTITY(1, 1) ,
      AccountID INT NOT NULL ,
      TransactionAmount DECIMAL(19, 2) NOT NULL
    )
GO

CREATE TRIGGER dbo.TRI_Transaction ON dbo.[Transaction]
    AFTER INSERT
AS
    UPDATE  a
    SET     a.AccountBalance = a.AccountBalance + i.TransactionAmount
    FROM    Account a
            JOIN INSERTED i ON ( a.AccountID = i.AccountID )
GO

INSERT  INTO dbo.Account
        ( AccountID, AccountBalance )
VALUES  ( 1234, 0 )

SELECT  *
FROM    dbo.Account a
        LEFT OUTER JOIN dbo.[Transaction] t ON ( a.AccountID = t.AccountID )

INSERT  INTO dbo.[Transaction]
        ( AccountID, TransactionAmount )
VALUES  ( 1234, 10 )

SELECT  *
FROM    dbo.Account a
        LEFT OUTER JOIN dbo.[Transaction] t ON ( a.AccountID = t.AccountID )


INSERT  INTO dbo.[Transaction]
        ( AccountID, TransactionAmount )
VALUES  ( 1234, 20 )

SELECT  *
FROM    dbo.Account a
        LEFT OUTER JOIN dbo.[Transaction] t ON ( a.AccountID = t.AccountID )

INSERT  INTO dbo.[Transaction]
        ( AccountID, TransactionAmount )
VALUES  ( 1234, -15 )


SELECT  *
FROM    dbo.Account a
        LEFT OUTER JOIN dbo.[Transaction] t ON ( a.AccountID = t.AccountID )

在行中输入行时，如何检查另一个表中的约束？

3 个答案: