Question

There is a way to create a constraint or something where I can have only one record for a Day for a FK column? I.E.

PK(IDENTITY(1,1))  FK_FIELD DATETIME
------------------------------
1                  1        2012-02-22 4:50 p.m.
2                  2        2012-02-23 12:00 p.m.
3                  2        2012-02-23 9:00 p.m.  -- This one should not be allowed because the FK_Field 2 would have 2 record in one day.
4                  1        2012-02-23 08:00 a.m 

Hopefully someone understand what im trying to archieve.

Thanks in advance.

Answer 1

Consider using:

• 1 or many persisted computed column(s)
• Unique constraint on the persisted computed column(s)

Some examples of a variety of persisted columns (Much thanks to Aaron Bertrand):

• Y AS YEAR(SOMEDATE) PERSISTED
• M AS MONTH(SOMEDATE) PERSISTED
• D AS MONTH(SOMEDATE) PERSISTED
• YMD AS YEAR(SOMEDATE) * 10000 + MONTH(SOMEDATE) * 100 + DAY(SOMEDATE) PERSISTED
• SOMEDATEDIFF AS DATEDIFF(DAY, 0, SOMEDATE) PERSISTED
• SOMEDATESMALL AS DATEADD(DAY, 0, DATEDIFF(DAY, 0, SOMEDATE)) PERSISTED
• SOMEDATESMALL AS CONVERT(DATETIME, FLOOR(CONVERT(FLOAT, SOMEDATE, 105)), 105) PERSISTED
• SOMEDATESMALL AS CONVERT(DATE, SOMEDATE) PERSISTED — Requires SQL Server 2008

N.B. These examples were chosen carefully to ensure function determinism.

Here’s a sample SQL:

CREATE TABLE DATA
(
     ID INT NOT NULL IDENTITY,
     FK_FIELD INT NOT NULL,
     SOMEDATE DATETIME NOT NULL,
     SOMEDATESMALL AS DATEADD(DAY, 0, DATEDIFF(DAY, 0, SOMEDATE)) PERSISTED,
     CONSTRAINT PK_DATA PRIMARY KEY (ID),
     CONSTRAINT UQ_DATA_SOMEDATESMALL UNIQUE (FK_FIELD, SOMEDATESMALL)
);

If you attempted the INSERTs, this is what you get:

INSERT INTO DATA (FK_FIELD, SOMEDATE) VALUES (1, '2012-02-22 4:50PM'); -- SUCCESS
INSERT INTO DATA (FK_FIELD, SOMEDATE) VALUES (2, '2012-02-23 12:00PM'); -- SUCCESS
INSERT INTO DATA (FK_FIELD, SOMEDATE) VALUES (2, '2012-02-23 9:00PM'); -- CAUGHT
INSERT INTO DATA (FK_FIELD, SOMEDATE) VALUES (1, '2012-02-23 8:00AM'); -- SUCCESS

The message shown in SQL Server Management Studio is:

Msg 2627, Level 14, State 1, Line 3
Violation of UNIQUE KEY constraint ‘UQ_DATA_SOMEDATESMALL’.
Cannot insert duplicate key in object ‘dbo.DATA’.
The statement has been terminated.