SQLServer如何保证可空字段中非空值唯一

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            <span><i class="fa fa-clock-o"></i><?php echo $website_info[0]["submission_date"];?></span>

include - ROOT/plugins/Kuhuang/templates/Websites/view.php, line 430
Cake\View\View::_evaluate() - CORE/src/View/View.php, line 1184
Cake\View\View::_render() - CORE/src/View/View.php, line 1138
Cake\View\View::render() - CORE/src/View/View.php, line 769
Cake\Controller\Controller::render() - CORE/src/Controller/Controller.php, line 762
App\Controller\NewsController::view() - APP/Controller/NewsController.php, line 3938
Cake\Controller\Controller::invokeAction() - CORE/src/Controller/Controller.php, line 539
Cake\Controller\ControllerFactory::handle() - CORE/src/Controller/ControllerFactory.php, line 140
Cake\Controller\ControllerFactory::invoke() - CORE/src/Controller/ControllerFactory.php, line 115
Cake\Http\BaseApplication::handle() - CORE/src/Http/BaseApplication.php, line 317
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 77
Cake\Http\Middleware\CsrfProtectionMiddleware::process() - CORE/src/Http/Middleware/CsrfProtectionMiddleware.php, line 164
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\I18n\Middleware\LocaleSelectorMiddleware::process() - CORE/src/I18n/Middleware/LocaleSelectorMiddleware.php, line 61
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\Http\Middleware\BodyParserMiddleware::process() - CORE/src/Http/Middleware/BodyParserMiddleware.php, line 157
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73

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            <span><i class="fa fa-clock-o"></i><?php echo $website_info[0]["submission_date"];?></span> 
include - ROOT/plugins/Kuhuang/templates/Websites/view.php, line 430
Cake\View\View::_evaluate() - CORE/src/View/View.php, line 1184
Cake\View\View::_render() - CORE/src/View/View.php, line 1138
Cake\View\View::render() - CORE/src/View/View.php, line 769
Cake\Controller\Controller::render() - CORE/src/Controller/Controller.php, line 762
App\Controller\NewsController::view() - APP/Controller/NewsController.php, line 3938
Cake\Controller\Controller::invokeAction() - CORE/src/Controller/Controller.php, line 539
Cake\Controller\ControllerFactory::handle() - CORE/src/Controller/ControllerFactory.php, line 140
Cake\Controller\ControllerFactory::invoke() - CORE/src/Controller/ControllerFactory.php, line 115
Cake\Http\BaseApplication::handle() - CORE/src/Http/BaseApplication.php, line 317
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 77
Cake\Http\Middleware\CsrfProtectionMiddleware::process() - CORE/src/Http/Middleware/CsrfProtectionMiddleware.php, line 164
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\I18n\Middleware\LocaleSelectorMiddleware::process() - CORE/src/I18n/Middleware/LocaleSelectorMiddleware.php, line 61
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\Http\Middleware\BodyParserMiddleware::process() - CORE/src/Http/Middleware/BodyParserMiddleware.php, line 157
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73

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            <span><i class="fa fa-clock-o"></i><?php echo $website_info[0]["submission_date"];?></span> 
include - ROOT/plugins/Kuhuang/templates/Websites/view.php, line 430
Cake\View\View::_evaluate() - CORE/src/View/View.php, line 1184
Cake\View\View::_render() - CORE/src/View/View.php, line 1138
Cake\View\View::render() - CORE/src/View/View.php, line 769
Cake\Controller\Controller::render() - CORE/src/Controller/Controller.php, line 762
App\Controller\NewsController::view() - APP/Controller/NewsController.php, line 3938
Cake\Controller\Controller::invokeAction() - CORE/src/Controller/Controller.php, line 539
Cake\Controller\ControllerFactory::handle() - CORE/src/Controller/ControllerFactory.php, line 140
Cake\Controller\ControllerFactory::invoke() - CORE/src/Controller/ControllerFactory.php, line 115
Cake\Http\BaseApplication::handle() - CORE/src/Http/BaseApplication.php, line 317
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 77
Cake\Http\Middleware\CsrfProtectionMiddleware::process() - CORE/src/Http/Middleware/CsrfProtectionMiddleware.php, line 164
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\I18n\Middleware\LocaleSelectorMiddleware::process() - CORE/src/I18n/Middleware/LocaleSelectorMiddleware.php, line 61
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\Http\Middleware\BodyParserMiddleware::process() - CORE/src/Http/Middleware/BodyParserMiddleware.php, line 157
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73

Warning (2): Trying to access array offset on value of type null [ROOT/plugins/Kuhuang/templates/Websites/view.php, line 430]Code Context          <div class="info"> 
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            <span><i class="fa fa-clock-o"></i><?php echo $website_info[0]["submission_date"];?></span> 
include - ROOT/plugins/Kuhuang/templates/Websites/view.php, line 430
Cake\View\View::_evaluate() - CORE/src/View/View.php, line 1184
Cake\View\View::_render() - CORE/src/View/View.php, line 1138
Cake\View\View::render() - CORE/src/View/View.php, line 769
Cake\Controller\Controller::render() - CORE/src/Controller/Controller.php, line 762
App\Controller\NewsController::view() - APP/Controller/NewsController.php, line 3938
Cake\Controller\Controller::invokeAction() - CORE/src/Controller/Controller.php, line 539
Cake\Controller\ControllerFactory::handle() - CORE/src/Controller/ControllerFactory.php, line 140
Cake\Controller\ControllerFactory::invoke() - CORE/src/Controller/ControllerFactory.php, line 115
Cake\Http\BaseApplication::handle() - CORE/src/Http/BaseApplication.php, line 317
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 77
Cake\Http\Middleware\CsrfProtectionMiddleware::process() - CORE/src/Http/Middleware/CsrfProtectionMiddleware.php, line 164
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\I18n\Middleware\LocaleSelectorMiddleware::process() - CORE/src/I18n/Middleware/LocaleSelectorMiddleware.php, line 61
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73
Cake\Http\Middleware\BodyParserMiddleware::process() - CORE/src/Http/Middleware/BodyParserMiddleware.php, line 157
Cake\Http\Runner::handle() - CORE/src/Http/Runner.php, line 73

表结构如下面代码创建复制代码代码如下:CREATETABLEtest_tb(TestIdintnotnullidentity(1,1)primarykey,Captionnvarchar(100)null);GO解决方案1：对于这个问题，大家的第一个想法可能是：在Caption这个字段上面加一个唯一键不就可以了吗？好，我们按着这个思路做下去，先创建唯一索引。复制代码代码如下:CREATEUNIQUENONCLUSTEREDINDEXun_test_tbONtest_tb(Caption)GO索引创建好了，我们来测试下效果复制代码代码如下:INSERTINTOtest_tb(Caption)VALUES(null)GOINSERTINTOtest_tb(Caption)VALUES(null)GO运行之后我们会收到下面的错误信息：以下为引用的内容：消息2601，级别14，状态1，第1行不能在具有唯一索引'un_test_tb'的对象'dbo.test_tb'中插入重复键的行

表结构如下面代码创建

复制代码代码如下:

 
CREATE TABLE test_tb 
( 
TestId int not null identity(1,1) primary key, 
Caption nvarchar(100) null 
); 
GO 

解决方案1：
对于这个问题，大家的第一个想法可能是：在Caption这个字段上面加一个唯一键不就可以了吗？好，我们按着这个思路做下去，先创建唯一索引。

复制代码代码如下:

 
CREATE UNIQUE NONCLUSTERED INDEX un_test_tb 
ON test_tb(Caption) 
GO 

索引创建好了，我们来测试下效果

复制代码代码如下:

 
INSERT INTO test_tb (Caption) 
VALUES (null) 
GO 
INSERT INTO test_tb (Caption) 
VALUES (null) 
GO 

运行之后我们会收到下面的错误信息：
以下为引用的内容：
消息 2601，级别 14，状态 1，第 1 行
不能在具有唯一索引 'un_test_tb' 的对象 'dbo.test_tb' 中插入重复键的行。
语句已终止。

所以该解决方案是不行的。
解决方案2：
添加约束，让SQL Server在插入数据的时候，先验证下已有数据中是否有现在要插入的这个值。由于这个约束不是简单的一个运算，因此我们先创建一个函数，然后再在约束中调用这个函数。
创建验证逻辑函数：

复制代码代码如下:

 
CREATE FUNCTION [dbo].[fn_CK_test_tb_Caption]() 
RETURNS BIT 
AS 
BEGIN 
IF(EXISTS( 
SELECT 1 
FROM test_tb AS a 
WHERE (Caption IS NOT NULL) AND EXISTS 
(SELECT 1 AS Expr1 
FROM test_tb 
WHERE (Caption IS NOT NULL) AND (Caption = a.Caption) AND (a.TestId <> TestId)) 
)) 
RETURN 0 
RETURN 1 
END 
GO 

在约束中引用函数：

复制代码代码如下:

 
ALTER TABLE test_tb 
ADD CONSTRAINT CK_test_tb_Caption CHECK (dbo.fn_CK_test_tb_Caption() = 1) 
GO 

现在来测试下效果。先来测试NULL值

复制代码代码如下:

 
INSERT INTO test_tb (Caption) 
VALUES (null) 
GO 
INSERT INTO test_tb (Caption) 
VALUES (null) 
GO 
SELECT * FROM test_tb 
GO 

可以成功运行，而且也出了多行为NULL的情况。现在再来测试不为空的插入情况。

复制代码代码如下:

 
INSERT INTO test_tb (Caption) 
VALUES (N'AAA') 
GO 
INSERT INTO test_tb (Caption) 
VALUES (N'BBB') 
GO 
INSERT INTO test_tb (Caption) 
VALUES (N'BBB') 
GO 
SELECT * FROM test_tb 
GO 

结果是在第三条语句的时候报错了，表中的Caption字段也有‘AAA'和‘BBB'了，这也正好是我们要的结果。
所以解决方案2是正确的。但是为了这么一个小小功能，就写这么长一段东西是不是太繁琐了呢？我们来看下面的解决方案。
解决方案3：（只适用于SQL Server 2008）
SQL Server 2008中有了一个优雅的解决方案，那就是筛选索引。筛选索引是一种经过优化的非聚集索引，尤其适用于涵盖从定义完善的数据子集中选择数据的查询。筛选索引使用筛选谓词对表中的部分行进行索引。有了筛选索引，我们只需要写一条语句就达到上面的效果。

复制代码代码如下:

 
CREATE UNIQUE NONCLUSTERED INDEX un_test_tb 
ON test_tb(Caption) 
WHERE Caption is not null 
GO 

再用上面的一些测试语句来测试的话，会发现完全是达到了我们的要求。这个方案的唯一缺点就是该语句只有SQL Server 2008支持。不知道各位有没有又优雅又适用于各个版本的SQL Server的解决方案，望不胜赐教。

标签: SQLServer 如何保证可空字段中非空值唯一

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