对于列A & B和A & C的唯一性的简单组合，可以使用唯一索引或唯一约束来解决这个问题。然而，对于后一个问题，这就不那么容易了。
我在这里使用了TRIGGER，并检查是否找到了具有相同值B和C的匹配行（其中ID不同，否则行将与其自身匹配），并且其中插入的行具有A的值2，或者现有行具有A的值。如果找到匹配，则错误为THROWn，从而导致INSERT/UPDATE发生故障。

CREATE TABLE dbo.YourTable (ID int IDENTITY,
                            A int NOT NULL,
                            B int NOT NULL,
                            C int NOT NULL);
GO

CREATE UNIQUE INDEX UX_YourTable_AB ON dbo.YourTable (A,B);
GO
CREATE UNIQUE INDEX UX_YourTable_AC ON dbo.YourTable (A,C);
GO

CREATE TRIGGER trg_chk_YourTable_BC_A2 ON dbo.YourTable
AFTER INSERT, UPDATE AS
BEGIN

    IF EXISTS (SELECT 1
               FROM inserted i
                    JOIN dbo.YourTable YT ON i.B = YT.B
                                         AND i.C = YT.C
                                         AND i.ID != YT.ID
                WHERE 2 IN (i.A,YT.A))
        THROW 54697, N'The check trigger ''trg_chk_YourTable_BC_A2'' on table ''dbo.YourTable'' failed. Cannot insert duplicate row on columns ''B'' and ''C'' where at least one row has the value of ''2''.',16;
END;
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(1,2,3);
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(1,2,4); --Fails
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(1,99,3); --Fails
GO
INSERT INTO dbo.YourTable (A,B,C)
VALUES(99,2,3); --Works
GO
INSERT INTO dbo.YourTable (A,B,C)
VALUES(2,2,3); --Fails
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(4,5,6); --Works

GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(5,5,6); --Works
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(2,5,6); --Fails
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(3,7,9); --Works
GO

INSERT INTO dbo.YourTable (A,B,C)
VALUES(2,7,9); --Fails

GO
SELECT *
FROM dbo.YourTable;

GO
DROP TABLE dbo.YourTable;

显然，您可能希望更改错误号和消息，但我写了一些合理的内容。db<>fiddle

粗略地看，我认为您可以使用唯一筛选索引。
1.唯一：A、B --始终
1.唯一：A、C --始终
1.唯一B、C，其中A==2

我会尝试使用公用表表达式并在插入语句中添加where子句。如果插入返回了零个更新的行，则认为插入失败？

-- Assuming A and B do not permit NULLs
; WITH cteAB AS (
    SELECT DISTINCT a,b FROM YourTable
)
, cteBC AS (
    SELECT DISTINCT b,c FROM YourTable
)
INSERT  YourTable ( a, b )
SELECT NewA, NewB
FROM YourTable yt 
LEFT JOIN cteAB ab ON ab.a=NewA AND ab.b=NewB
LEFT JOIN cteBC bc ON bc.a=NewA AND bc.b=NewB
WHERE ( ab.a IS NULL AND bc.a IS NULL )
    OR NewA=2