使列基於另一列的值接收0或1

Question

我想知道是否可以在表中創建列，並根據同一表中另一列的值自動獲取它們的值，例如以下澄清：

CREATE TABLE dbo.example 
(
    m_id INT NOT NULL CONSTRAINT PK_mid PRIMARY KEY IDENTITY (1,1), 
    m_name NVARCHAR(30) NOT NULL, 
    m_startdate DATE NOT NULL CONSTRAINT CHK_startdate CHECK(m_startdate <= SYSDATETIME()), 
    m_enddate DATE CONSTRAINT CHK_enddate CHECK(m_enddate <= SYSDATETIME()), 
    m_status INT CONSTRAINT CHK_status CHECK(m_status = 0 or m_status = 1) 
) 

我要讓m_status接受0，如果m_enddate爲null 1，如果它不爲空。這當然會在插入一行。

Answer 1

如下你可以使用一個計算列...

CREATE TABLE dbo.example 
(
m_id INT NOT NULL CONSTRAINT PK_mid PRIMARY KEY IDENTITY (1,1), 
m_name NVARCHAR(30) NOT NULL, 
m_startdate DATE NOT NULL CONSTRAINT CHK_startdate CHECK(m_startdate <= SYSDATETIME()), 
m_enddate DATE CONSTRAINT CHK_enddate CHECK(m_enddate <= SYSDATETIME()), 
m_status AS CASE 
    WHEN m_enddate is null THEN 0 ELSE 1 
    END 
) 

Answer 2

計算列邁克爾建議是最好的，但如果你不能改變的表，那麼你可以彌補這方面的視圖。現在

CREATE VIEW dbo.vwExample AS 
    SELECT m_id, 
     m_name, 
     m_startdate, 
     m_enddate, 
     CASE WHEN m_enddate is null THEN 0 ELSE 1 END as m_status 
    FROM dbo.example 

你可以做

select * from dbo.vwExample 

，這將有m_status正確的值，而不必更改表本身。

Answer 3

在T-SQL中，您可以使用替代插入/更新觸發器，它允許您攔截插入和更新並注入邏輯。

CREATE TRIGGER dbo.exampleInsertTrigger 
    ON [dbo].[example] 
    INSTEAD OF INSERT 
AS 
BEGIN 
INSERT INTO [dbo].[example] 
    ([m_name] 
    ,[m_startdate] 
    ,[m_enddate] 
    ,[m_status]) 
SELECT 
    [m_name] 
    ,[m_startdate] 
    ,[m_enddate] 
    ,CASE 
    WHEN [m_enddate] is null THEN 0 ELSE 1 
    END 
FROM inserted 
END 


CREATE TRIGGER dbo.exampleUpdateTrigger 
    ON dbo.example 
    INSTEAD OF UPDATE 
AS 
BEGIN 
UPDATE [dbo].[example] 
    SET 
    [m_name] = inserted.[m_name] 
    ,[m_startdate] = inserted.[m_startdate] 
    ,[m_enddate] = inserted.[m_enddate] 
    ,[m_status] = CASE 
    WHEN inserted.[m_enddate] is null THEN 0 ELSE 1 
    END 
FROM inserted 
WHERE inserted.[m_id] = [dbo].[example].[m_id] 
END