SQL约束在两列中有一个唯一值

Question

我需要确保两列中的值是唯一的（这不是“两列”索引问题）。

Table A 
Column A1  Column A2 

Memphis   New York  -> ok 
San Francisco Miami  -> ok 
Washington  Chicago  -> ok 
Miami   Las Vegas -> Forbidden ! Miami already exists 

这可能吗？

我的例子是与城市，但不集中在那。我真正的需要是关于生成的十六进制ID。

Answer 1

您需要添加一个约束触发器，在插入/更新后查找它。

Answer 2

在SQL Server中，可以通过索引视图的帮助实施唯一性。你还需要一个数字表（如果你还没有的话）与你的Table A在同一个数据库中。

这里是我的测试脚本一些意见：

CREATE TABLE MyNumbersTable (Value int); 
-- You need at least 2 rows, by the number of columns 
-- you are going to implement uniqueness on 
INSERT INTO MyNumbersTable 
SELECT 1 UNION ALL 
SELECT 2; 
GO 
CREATE TABLE MyUniqueCities ( -- the main table 
    ID int IDENTITY, 
    City1 varchar(50) NOT NULL, 
    City2 varchar(50) NOT NULL 
); 
GO 
CREATE VIEW MyIndexedView 
WITH SCHEMABINDING -- this is required for creating an indexed view 
AS 
SELECT 
    City = CASE t.Value -- after supplying the numbers table 
    WHEN 1 THEN u.City1 -- with the necessary number of rows 
    WHEN 2 THEN u.City2 -- you can extend this CASE expression 
    END     -- to add more columns to the constraint 
FROM dbo.MyUniqueCities u 
    INNER JOIN dbo.MyNumbersTable t 
    ON t.Value BETWEEN 1 AND 2 -- change here too for more columns 
GO 
-- the first index on an indexed view *must* be unique, 
-- which suits us perfectly 
CREATE UNIQUE CLUSTERED INDEX UIX_MyIndexedView ON MyIndexedView (City) 
GO 
-- the first two rows insert fine 
INSERT INTO MyUniqueCities VALUES ('London', 'New York'); 
INSERT INTO MyUniqueCities VALUES ('Amsterdam', 'Prague'); 
-- the following insert produces an error, because of 'London' 
INSERT INTO MyUniqueCities VALUES ('Melbourne', 'London'); 
GO 
DROP VIEW MyIndexedView 
DROP TABLE MyUniqueCities 
DROP TABLE MyNumbersTable 
GO 

有用的书：

• Creating Indexed Views

• The "Numbers" or "Tally" Table: What it is and how it replaces a loop.