先检查加约束，然后再检查对比与增加约束

我正在查看SQL Server 2008的AdventureWorks示例数据库，并且在他们的创建脚本中看到他们倾向于使用以下内容：

ALTER TABLE [Production].[ProductCostHistory] WITH CHECK ADD 
CONSTRAINT [FK_ProductCostHistory_Product_ProductID] FOREIGN KEY([ProductID])
  REFERENCES [Production].[Product] ([ProductID])
GO

紧随其后的是：

ALTER TABLE [Production].[ProductCostHistory] CHECK CONSTRAINT     
[FK_ProductCostHistory_Product_ProductID]
GO

我在外键（如此处），唯一约束和常规CHECK约束中看到了这一点；DEFAULT约束使用我更熟悉的常规格式，例如：

ALTER TABLE [Production].[ProductCostHistory] ADD  CONSTRAINT  
[DF_ProductCostHistory_ModifiedDate]  DEFAULT (getdate()) FOR [ModifiedDate]
GO

第一种方法与第二种方法有什么区别（如果有）？

第一种语法是多余的-WITH CHECK是新约束的默认设置，并且该约束在默认情况下也是打开的。

这种语法是由SQL Management Studio在生成sql脚本时生成的-我假设它具有某种额外的冗余性，以确保即使更改了表的默认约束行为，也可以确保启用了约束。

为了演示其工作原理-

CREATE TABLE T1 (ID INT NOT NULL, SomeVal CHAR(1));
ALTER TABLE T1 ADD CONSTRAINT [PK_ID] PRIMARY KEY CLUSTERED (ID);

CREATE TABLE T2 (FKID INT, SomeOtherVal CHAR(2));

INSERT T1 (ID, SomeVal) SELECT 1, 'A';
INSERT T1 (ID, SomeVal) SELECT 2, 'B';

INSERT T2 (FKID, SomeOtherVal) SELECT 1, 'A1';
INSERT T2 (FKID, SomeOtherVal) SELECT 1, 'A2';
INSERT T2 (FKID, SomeOtherVal) SELECT 2, 'B1';
INSERT T2 (FKID, SomeOtherVal) SELECT 2, 'B2';
INSERT T2 (FKID, SomeOtherVal) SELECT 3, 'C1';  --orphan
INSERT T2 (FKID, SomeOtherVal) SELECT 3, 'C2';  --orphan

--Add the FK CONSTRAINT will fail because of existing orphaned records
ALTER TABLE T2 ADD CONSTRAINT FK_T2_T1 FOREIGN KEY (FKID) REFERENCES T1 (ID);   --fails

--Same as ADD above, but explicitly states the intent to CHECK the FK values before creating the CONSTRAINT
ALTER TABLE T2 WITH CHECK ADD CONSTRAINT FK_T2_T1 FOREIGN KEY (FKID) REFERENCES T1 (ID);    --fails

--Add the CONSTRAINT without checking existing values
ALTER TABLE T2 WITH NOCHECK ADD CONSTRAINT FK_T2_T1 FOREIGN KEY (FKID) REFERENCES T1 (ID);  --succeeds
ALTER TABLE T2 CHECK CONSTRAINT FK_T2_T1;   --succeeds since the CONSTRAINT is attributed as NOCHECK

--Attempt to enable CONSTRAINT fails due to orphans
ALTER TABLE T2 WITH CHECK CHECK CONSTRAINT FK_T2_T1;    --fails

--Remove orphans
DELETE FROM T2 WHERE FKID NOT IN (SELECT ID FROM T1);

--Enabling the CONSTRAINT succeeds
ALTER TABLE T2 WITH CHECK CHECK CONSTRAINT FK_T2_T1;    --succeeds; orphans removed

--Clean up
DROP TABLE T2;
DROP TABLE T1;

除了上述有关受信任约束的出色评论之外：

select * from sys.foreign_keys where is_not_trusted = 1 ;
select * from sys.check_constraints where is_not_trusted = 1 ;

顾名思义，一个不受信任的约束现在不能被信任以准确表示表中数据的状态。但是，它可以但将来可以用来检查添加和修改的数据。

此外，查询优化程序会忽略不受信任的约束。

用于启用检查约束和外键约束的代码非常糟糕，具有“检查”一词的三个含义。

ALTER TABLE [Production].[ProductCostHistory] 
WITH CHECK -- This means "Check the existing data in the table".
CHECK CONSTRAINT -- This means "enable the check or foreign key constraint".
[FK_ProductCostHistory_Product_ProductID] -- The name of the check or foreign key constraint, or "ALL".

WITH NOCHECK 当一个表中的现有数据不符合所定义的约束并且您不希望它与正在实现的新约束冲突时，也会使用。

WITH CHECK 确实是默认行为，但是在您的编码中包含它是一种好习惯。

当然可以使用替代行为WITH NOCHECK，因此最好明确定义您的意图。当您在使用/修改/切换嵌入式分区时，通常会使用它。

外键和检查约束具有被信任或不受信任以及被启用和禁用的概念。请参阅MSDN页面以获取ALTER TABLE完整的详细信息，。

WITH CHECK 是添加新外键和检查约束的默认设置， WITH NOCHECK的默认值是重新启用禁用的外键和检查约束的默认值。意识到差异很重要。

话虽如此，实用程序生成的任何显然多余的语句只是为了安全和/或简化编码而存在。不用担心

这是我编写的一些代码，以帮助我们识别和更正数据库中不受信任的约束。它生成用于修复每个问题的代码。

    ;WITH Untrusted (ConstraintType, ConstraintName, ConstraintTable, ParentTable, IsDisabled, IsNotForReplication, IsNotTrusted, RowIndex) AS
(
    SELECT 
        'Untrusted FOREIGN KEY' AS FKType
        , fk.name AS FKName
        , OBJECT_NAME( fk.parent_object_id) AS FKTableName
        , OBJECT_NAME( fk.referenced_object_id) AS PKTableName 
        , fk.is_disabled
        , fk.is_not_for_replication
        , fk.is_not_trusted
        , ROW_NUMBER() OVER (ORDER BY OBJECT_NAME( fk.parent_object_id), OBJECT_NAME( fk.referenced_object_id), fk.name) AS RowIndex
    FROM 
        sys.foreign_keys fk 
    WHERE 
        is_ms_shipped = 0 
        AND fk.is_not_trusted = 1       

    UNION ALL

    SELECT 
        'Untrusted CHECK' AS KType
        , cc.name AS CKName
        , OBJECT_NAME( cc.parent_object_id) AS CKTableName
        , NULL AS ParentTable
        , cc.is_disabled
        , cc.is_not_for_replication
        , cc.is_not_trusted
        , ROW_NUMBER() OVER (ORDER BY OBJECT_NAME( cc.parent_object_id), cc.name) AS RowIndex
    FROM 
        sys.check_constraints cc 
    WHERE 
        cc.is_ms_shipped = 0
        AND cc.is_not_trusted = 1

)
SELECT 
    u.ConstraintType
    , u.ConstraintName
    , u.ConstraintTable
    , u.ParentTable
    , u.IsDisabled
    , u.IsNotForReplication
    , u.IsNotTrusted
    , u.RowIndex
    , 'RAISERROR( ''Now CHECKing {%i of %i)--> %s ON TABLE %s'', 0, 1' 
        + ', ' + CAST( u.RowIndex AS VARCHAR(64))
        + ', ' + CAST( x.CommandCount AS VARCHAR(64))
        + ', ' + '''' + QUOTENAME( u.ConstraintName) + '''' 
        + ', ' + '''' + QUOTENAME( u.ConstraintTable) + '''' 
        + ') WITH NOWAIT;'
    + 'ALTER TABLE ' + QUOTENAME( u.ConstraintTable) + ' WITH CHECK CHECK CONSTRAINT ' + QUOTENAME( u.ConstraintName) + ';' AS FIX_SQL
FROM Untrusted u
CROSS APPLY (SELECT COUNT(*) AS CommandCount FROM Untrusted WHERE ConstraintType = u.ConstraintType) x
ORDER BY ConstraintType, ConstraintTable, ParentTable;