我们如何创建一个表约束，将一列限制为x个相同值？

这并不理想，但是正如我在评论中提到的那样，我将在执行INSERT/ UPDATE/ 的SP中处理此问题DELETE（不太可能需要DELETE）。因此，对于，INSERT您将具有如下所示的内容：

USE Sandbox; GO

CREATE TABLE [dbo].[BasketProduct](
    [BasketProductId] [int] IDENTITY(1,1) NOT NULL,
    [BasketId] [int] NOT NULL,
    [ProductId] [int] NOT NULL,
    [Quantity] [int] NOT NULL,
 CONSTRAINT [PK_BasketProduct] PRIMARY KEY CLUSTERED 
([BasketProductId] ASC));
GO

CREATE PROC AddBasketProduct @BasketID int, @ProductID int, @Quantity int AS
BEGIN

    DECLARE @DistinctProducts int;

    SELECT @DistinctProducts = COUNT(DISTINCT ProductID)
    FROM dbo.BasketProduct WITH (UPDLOCK) --As we need to control concurrency issues
    WHERE BasketId = @BasketID
      AND ProductID != @ProductID ;

    IF @DistinctProducts >= 10
        THROW 71245, N'Cannot have more than 10 different products in a single basket.',16; --Choose an error number and state appropraite for your applciation
    ELSE
        INSERT INTO dbo.BasketProduct (BasketId,
                                       ProductId,
                                       Quantity)
        VALUES(@BasketID,@ProductID,@Quantity);
END;

GO
--Make some sample data
INSERT INTO dbo.BasketProduct (BasketId,
                               ProductId,
                               Quantity)
VALUES(1,1,1),
      (1,2,1),
      (1,3,1),
      (1,4,1),
      (1,5,1),
      (1,6,1),
      (1,7,1),
      (1,8,1),
      (1,9,1),
      (1,10,1); --10 products.
GO
--11th product, will fail
EXEC dbo.AddBasketProduct @BasketID = 1,
                          @ProductID = 11,
                          @Quantity = 1;

GO

--Repetition of product 2, will work
EXEC dbo.AddBasketProduct @BasketID = 1,
                          @ProductID = 2,
                          @Quantity = 1;
GO

DROP PROC dbo.AddBasketProduct;
DROP TABLE dbo.BasketProduct;