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I'm having trouble with the design of a temporal database. I need to know how to make sure I have only one active record for any given timeframe for a store. I have read this answer, but I'm afraid I can't wrap my head around how the trigger would work. Particularly, how I would work that trigger into my existing one that prevents updates to records, and inserts a new record instead. My real problem is that I do not know how to prevent a Store from having more than one effective date when the finished date is null. (i.e. prevent 2 active records for a store).

This is what I have, but it allows me to insert a new record for a store with a different effective date.

Table Definition:

/****** Object:  Table [PCR].[Z_STORE_TEAM]    Script Date: 05/09/2014 13:05:57 ******/
IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[Z_STORE_TEAM]') AND type in (N'U'))
DROP TABLE [Z_STORE_TEAM]
GO

IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[Z_STORE_TEAM]') AND type in (N'U'))
BEGIN
CREATE TABLE [Z_STORE_TEAM](
    [STORENUM] [int] NOT NULL,
    [TEAM] [varchar](10) NULL,
    [EFFECTIVE] [date] NOT NULL,
    [FINISHED] [date] NULL,
PRIMARY KEY CLUSTERED 
(
    [STORENUM] ASC,
    [EFFECTIVE] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
END
GO

Sample Data:

INSERT [Z_STORE_TEAM] ([STORENUM], [TEAM], [EFFECTIVE], [FINISHED]) VALUES (1, N'1', CAST(0x01380B00 AS Date), CAST(0x81380B00 AS Date))
INSERT [Z_STORE_TEAM] ([STORENUM], [TEAM], [EFFECTIVE], [FINISHED]) VALUES (1, N'2', CAST(0x81380B00 AS Date), NULL)
INSERT [Z_STORE_TEAM] ([STORENUM], [TEAM], [EFFECTIVE], [FINISHED]) VALUES (2, N'1', CAST(0x01380B00 AS Date), NULL)
INSERT [Z_STORE_TEAM] ([STORENUM], [TEAM], [EFFECTIVE], [FINISHED]) VALUES (2, N'2', CAST(0x20380B00 AS Date), NULL)

Instead of Update Trigger:

CREATE TRIGGER [tr_ZStoreTeam_update] 
   ON  [Z_STORE_TEAM]
   INSTEAD OF UPDATE
AS 
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    -- Insert statements for trigger here
    INSERT INTO PCR.Z_STORE_TEAM(STORENUM,TEAM,EFFECTIVE)
    SELECT I.STORENUM,I.TEAM,GETDATE() AS EFFECTIVE
    FROM inserted I
    INNER JOIN PCR.Z_STORE_TEAM ST
        ON I.STORENUM = ST.STORENUM

    UPDATE ST
    SET FINISHED = GETDATE()
    FROM PCR.Z_STORE_TEAM ST
    INNER JOIN inserted I
        ON ST.STORENUM = I.STORENUM
        AND ST.EFFECTIVE = I.EFFECTIVE
END

GO
share|improve this question
    
Using a trigger for this might be problematic if the user were to INSERT multiple rows in a single statement. Would you fail the entire INSERT because a single row wasn't valid? Would you silently fail the single row, while allowing the valid rows to be inserted? –  Max Vernon May 9 at 17:32
    
@MaxVernon Fair enough. I hadn't thought about that. I've never had this kind of history requirement before, so I'm all ears to better solutions to this problem. I can't be the first person to face this problem. –  RubberDuck May 9 at 17:35
    
You could create a stored procedure that runs the inserts. The stored proc could check for the existence of a prior row, and fail if one exists. Rights to insert on the table could be denied to everyone except the stored proc. The stored proc could be programmed to return a resultset of failed rows, if desired, instead of failing completely. Depends on your requirements. –  Max Vernon May 9 at 18:09

1 Answer 1

up vote 8 down vote accepted

The safest way to do this is to enforce your business rules using the built-in referential integrity constraints, as Alexander Kuznetsov describes in his article, "Storing intervals of time with no overlaps".

Applying the techniques listed there to your sample table results in the following script:

CREATE TABLE [Z_STORE_TEAM](
    [STORENUM] [int] NOT NULL,
    [TEAM] [varchar](10) NULL,
    [EFFECTIVE] [date] NOT NULL,
    [FINISHED] [date] NULL,
    PRIMARY KEY CLUSTERED 
    (
        [STORENUM] ASC,
        [EFFECTIVE] ASC
    )
) ON [PRIMARY];

INSERT [Z_STORE_TEAM] 
    ([STORENUM], [TEAM], [EFFECTIVE], [FINISHED]) 
VALUES 
    (1, N'1', CAST(0x01380B00 AS Date), CAST(0x81380B00 AS Date)),
    (1, N'2', CAST(0x81380B00 AS Date), NULL),
    (2, N'1', CAST(0x01380B00 AS Date), NULL);

Modifications:

-- New column to hold the previous finish date
ALTER TABLE dbo.Z_STORE_TEAM 
ADD PreviousFinished date NULL;
GO
-- Populate the previous finish date
UPDATE This
SET PreviousFinished = Previous.FINISHED
FROM dbo.Z_STORE_TEAM AS This
CROSS APPLY
(
    SELECT TOP (1) 
        Previous.FINISHED
    FROM dbo.Z_STORE_TEAM AS Previous
    WHERE 
        Previous.STORENUM = This.STORENUM
        AND Previous.FINISHED <= This.EFFECTIVE
    ORDER BY 
        Previous.FINISHED DESC
) AS Previous;
GO
ALTER TABLE dbo.Z_STORE_TEAM 
ADD CONSTRAINT UQ_STORENUM_PreviousFinished
UNIQUE (STORENUM, PreviousFinished);
GO
ALTER TABLE dbo.Z_STORE_TEAM
ADD CONSTRAINT CK_PreviousFinished_NotAfter_Effective
CHECK (PreviousFinished = EFFECTIVE);
GO
ALTER TABLE dbo.Z_STORE_TEAM
ADD CONSTRAINT UQ_STORENUM_FINISHED
UNIQUE (STORENUM, FINISHED);
GO
ALTER TABLE dbo.Z_STORE_TEAM
ADD CONSTRAINT FK_STORENUM_PreviousFinished
FOREIGN KEY (STORENUM, PreviousFinished)
REFERENCES dbo.Z_STORE_TEAM (STORENUM, FINISHED);
GO
ALTER TABLE dbo.Z_STORE_TEAM
ADD CONSTRAINT CK_EFFECTIVE_Before_FINISHED
CHECK (EFFECTIVE < FINISHED);

An attempt to insert the fourth row of sample data now fails with an error message:

INSERT [Z_STORE_TEAM] 
    ([STORENUM], [TEAM], [EFFECTIVE], [FINISHED]) 
VALUES 
    (2, N'2', '20140201', NULL);

Error message

Please read Alex's article to understand how these constraints ensure your table data will always be valid. Having a set of constraints enforce your data integrity means no trigger code is required.

share|improve this answer
    
Thanks Paul. I'll look at this later today. –  RubberDuck May 12 at 12:15
    
How would you go about splitting an existing (closed) range into two with this restriction implemented? Doesn't seem possible to me, but I might be missing something. (However, it certainly seems possible with the other variation, the one that allows gaps.) –  Andriy M Jul 23 at 9:08
1  
@AndriyM See the comments on Alex's article, re: using MERGE. –  Paul White Jul 23 at 9:22

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