leora leora - 3 months ago 16
SQL Question

Add unique constraint to combination of two columns

I have a table and somehow the same person got into my

Person
table twice. Right now the primary key is just the autonumber but there are two other fields that exist that I want to force to be unique.

For example, the fields are:

ID
Name
Active
PersonNumber


I only want 1 record with a unique PersonNumber and Active = 1

(So the combination of the two fields needs to be unique)

What is the best way on an existing table in SQL server I can make so if anyone else does an insert with the same value as an existing value, it fails so I don't have to worry about this in my application code.

Answer

Once you have removed your duplicate(s):

ALTER TABLE dbo.yourtablename
  ADD CONSTRAINT uq_yourtablename UNIQUE(column1, column2);

or

CREATE UNIQUE INDEX uq_yourtablename
  ON dbo.yourtablename(column1, column2);

Of course, it can often be better to check for this violation first, before just letting SQL Server try to insert the row and returning an exception (exceptions are expensive).

http://www.sqlperformance.com/2012/08/t-sql-queries/error-handling

http://www.mssqltips.com/sqlservertip/2632/checking-for-potential-constraint-violations-before-entering-sql-server-try-and-catch-logic/

If you want to prevent exceptions from bubbling up to the application, without making changes to the application, you can use an INSTEAD OF trigger:

CREATE TRIGGER dbo.BlockDuplicatesYourTable
 ON dbo.YourTable
 INSTEAD OF INSERT
AS
BEGIN
  SET NOCOUNT ON;

  IF NOT EXISTS (SELECT 1 FROM inserted AS i 
    INNER JOIN dbo.YourTable AS t
    ON i.column1 = t.column1
    AND i.column2 = t.column2
  )
  BEGIN
    INSERT dbo.YourTable(column1, column2, ...)
      SELECT column1, column2, ... FROM inserted;
  END
  ELSE
  BEGIN
    PRINT 'Did nothing.';
  END
END
GO

But if you don't tell the user they didn't perform the insert, they're going to wonder why the data isn't there and no exception was reported.


EDIT here is an example that does exactly what you're asking for, even using the same names as your question, and proves it. You should try it out before assuming the above ideas only treat one column or the other as opposed to the combination...

USE tempdb;
GO

CREATE TABLE dbo.Person
(
  ID INT IDENTITY(1,1) PRIMARY KEY,
  Name NVARCHAR(32),
  Active BIT,
  PersonNumber INT
);
GO

ALTER TABLE dbo.Person 
  ADD CONSTRAINT uq_Person UNIQUE(PersonNumber, Active);
GO

-- succeeds:
INSERT dbo.Person(Name, Active, PersonNumber)
  VALUES(N'foo', 1, 22);
GO

-- succeeds:
INSERT dbo.Person(Name, Active, PersonNumber)
  VALUES(N'foo', 0, 22);
GO

-- fails:
INSERT dbo.Person(Name, Active, PersonNumber)
  VALUES(N'foo', 1, 22);
GO

Data in the table after all of this:

ID   Name   Active PersonNumber
---- ------ ------ ------------
1    foo    1      22
2    foo    0      22

Error message on the last insert:

Msg 2627, Level 14, State 1, Line 3 Violation of UNIQUE KEY constraint 'uq_Person'. Cannot insert duplicate key in object 'dbo.Person'. The statement has been terminated.

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