几乎可以肯定，检查是必须的!

在一些回答和评论中提出了这一点，但我认为再次提出这一点非常重要。

使用以下命令(不使用WITH CHECK)重新启用约束会有一些严重的缺陷。

ALTER TABLE MyTable CHECK CONSTRAINT MyConstraint;

WITH CHECK | WITH NOCHECK Specifies whether the data in the table is or is not validated against a newly added or re-enabled FOREIGN KEY or CHECK constraint. If not specified, WITH CHECK is assumed for new constraints, and WITH NOCHECK is assumed for re-enabled constraints. If you do not want to verify new CHECK or FOREIGN KEY constraints against existing data, use WITH NOCHECK. We do not recommend doing this, except in rare cases. The new constraint will be evaluated in all later data updates. Any constraint violations that are suppressed by WITH NOCHECK when the constraint is added may cause future updates to fail if they update rows with data that does not comply with the constraint. The query optimizer does not consider constraints that are defined WITH NOCHECK. Such constraints are ignored until they are re-enabled by using ALTER TABLE table WITH CHECK CHECK CONSTRAINT ALL.

注意:WITH NOCHECK是重新启用约束的默认值。我想知道为什么……

No existing data in the table will be evaluated during the execution of this command - successful completion is no guarantee that the data in the table is valid according to the constraint. During the next update of the invalid records, the constraint will be evaluated and will fail - resulting in errors that may be unrelated to the actual update that is made. Application logic that relies on the constraint to ensure that data is valid may fail. The query optimizer will not make use of any constraint that is enabled in this way.

sys。Foreign_keys系统视图提供了一些问题的可见性。注意，它有一个is_disabled列和一个is_not_trusted列。Is_disabled指示是否根据约束验证未来的数据操作。Is_not_trusted指示表中当前的所有数据是否已经根据约束进行了验证。

ALTER TABLE MyTable WITH CHECK CHECK CONSTRAINT MyConstraint;

你的约束是可信的吗?找出……

SELECT * FROM sys.foreign_keys WHERE is_not_trusted = 1;

SET NOCOUNT ON

DECLARE @table TABLE(
   RowId INT PRIMARY KEY IDENTITY(1, 1),
   ForeignKeyConstraintName NVARCHAR(200),
   ForeignKeyConstraintTableSchema NVARCHAR(200),
   ForeignKeyConstraintTableName NVARCHAR(200),
   ForeignKeyConstraintColumnName NVARCHAR(200),
   PrimaryKeyConstraintName NVARCHAR(200),
   PrimaryKeyConstraintTableSchema NVARCHAR(200),
   PrimaryKeyConstraintTableName NVARCHAR(200),
   PrimaryKeyConstraintColumnName NVARCHAR(200),
   UpdateRule NVARCHAR(100),
   DeleteRule NVARCHAR(100)   
)

INSERT INTO @table(ForeignKeyConstraintName, ForeignKeyConstraintTableSchema, ForeignKeyConstraintTableName, ForeignKeyConstraintColumnName)
SELECT 
   U.CONSTRAINT_NAME, 
   U.TABLE_SCHEMA, 
   U.TABLE_NAME, 
   U.COLUMN_NAME
FROM 
   INFORMATION_SCHEMA.KEY_COLUMN_USAGE U
      INNER JOIN INFORMATION_SCHEMA.TABLE_CONSTRAINTS C
         ON U.CONSTRAINT_NAME = C.CONSTRAINT_NAME
WHERE
   C.CONSTRAINT_TYPE = 'FOREIGN KEY'

UPDATE @table SET
   T.PrimaryKeyConstraintName = R.UNIQUE_CONSTRAINT_NAME,
   T.UpdateRule = R.UPDATE_RULE,
   T.DeleteRule = R.DELETE_RULE
FROM 
   @table T
      INNER JOIN INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS R
         ON T.ForeignKeyConstraintName = R.CONSTRAINT_NAME

UPDATE @table SET
   PrimaryKeyConstraintTableSchema  = TABLE_SCHEMA,
   PrimaryKeyConstraintTableName  = TABLE_NAME
FROM @table T
   INNER JOIN INFORMATION_SCHEMA.TABLE_CONSTRAINTS C
      ON T.PrimaryKeyConstraintName = C.CONSTRAINT_NAME

UPDATE @table SET
   PrimaryKeyConstraintColumnName = COLUMN_NAME
FROM @table T
   INNER JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE U
      ON T.PrimaryKeyConstraintName = U.CONSTRAINT_NAME

--SELECT * FROM @table

SELECT '
BEGIN TRANSACTION
BEGIN TRY'

--DROP CONSTRAINT:
SELECT
   '
 ALTER TABLE [' + ForeignKeyConstraintTableSchema + '].[' + ForeignKeyConstraintTableName + '] 
 DROP CONSTRAINT ' + ForeignKeyConstraintName + '
   '
FROM
   @table

SELECT '
END TRY

BEGIN CATCH
   ROLLBACK TRANSACTION
   RAISERROR(''Operation failed.'', 16, 1)
END CATCH

IF(@@TRANCOUNT != 0)
BEGIN
   COMMIT TRANSACTION
   RAISERROR(''Operation completed successfully.'', 10, 1)
END
'

--ADD CONSTRAINT:
SELECT '
BEGIN TRANSACTION
BEGIN TRY'

SELECT
   '
   ALTER TABLE [' + ForeignKeyConstraintTableSchema + '].[' + ForeignKeyConstraintTableName + '] 
   ADD CONSTRAINT ' + ForeignKeyConstraintName + ' FOREIGN KEY(' + ForeignKeyConstraintColumnName + ') REFERENCES [' + PrimaryKeyConstraintTableSchema + '].[' + PrimaryKeyConstraintTableName + '](' + PrimaryKeyConstraintColumnName + ') ON UPDATE ' + UpdateRule + ' ON DELETE ' + DeleteRule + '
   '
FROM
   @table

SELECT '
END TRY

BEGIN CATCH
   ROLLBACK TRANSACTION
   RAISERROR(''Operation failed.'', 16, 1)
END CATCH

IF(@@TRANCOUNT != 0)
BEGIN
   COMMIT TRANSACTION
   RAISERROR(''Operation completed successfully.'', 10, 1)
END'

GO

如果你想禁用数据库中的所有约束，只需运行以下代码:

-- disable all constraints
EXEC sp_MSforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all"

要将它们重新打开，运行:(打印当然是可选的，它只是列出表)

-- enable all constraints
exec sp_MSforeachtable @command1="print '?'", @command2="ALTER TABLE ? WITH CHECK CHECK CONSTRAINT all"

我发现它在将数据从一个数据库填充到另一个数据库时很有用。这是比放弃约束更好的方法。正如您所提到的，当删除数据库中的所有数据并重新填充它时(例如在测试环境中)，它非常方便。

如果您正在删除所有数据，您可能会发现这个解决方案是有用的。

有时禁用所有触发器也很方便，你可以在这里看到完整的解决方案。

找到约束条件

SELECT * 
FROM sys.foreign_keys
WHERE referenced_object_id = object_id('TABLE_NAME')

执行由此SQL生成的SQL

SELECT 
    'ALTER TABLE ' +  OBJECT_SCHEMA_NAME(parent_object_id) +
    '.[' + OBJECT_NAME(parent_object_id) + 
    '] DROP CONSTRAINT ' + name
FROM sys.foreign_keys
WHERE referenced_object_id = object_id('TABLE_NAME')

西夫韦。

注意:增加了删除约束的解决方案，这样可以删除或修改表而没有任何约束错误。

最好的选择是DROP和CREATE外键约束。

我在这篇文章中没有找到对我“原样”工作的例子，如果外键引用不同的模式，一个不会工作，如果外键引用多列，另一个也不会工作。这个脚本同时考虑多个模式和每个外键的多个列。

下面是生成“ADD CONSTRAINT”语句的脚本，对于多列，它将用逗号分隔它们(确保在执行DROP语句之前保存此输出):

PRINT N'-- CREATE FOREIGN KEY CONSTRAINTS --';

SET NOCOUNT ON;
SELECT '
PRINT N''Creating '+ const.const_name +'...''
GO
ALTER TABLE ' + const.parent_obj + '
    ADD CONSTRAINT ' + const.const_name + ' FOREIGN KEY (
            ' + const.parent_col_csv + '
            ) REFERENCES ' + const.ref_obj + '(' + const.ref_col_csv + ')
GO'
FROM (
    SELECT QUOTENAME(fk.NAME) AS [const_name]
        ,QUOTENAME(schParent.NAME) + '.' + QUOTENAME(OBJECT_name(fkc.parent_object_id)) AS [parent_obj]
        ,STUFF((
                SELECT ',' + QUOTENAME(COL_NAME(fcP.parent_object_id, fcp.parent_column_id))
                FROM sys.foreign_key_columns AS fcP
                WHERE fcp.constraint_object_id = fk.object_id
                FOR XML path('')
                ), 1, 1, '') AS [parent_col_csv]
        ,QUOTENAME(schRef.NAME) + '.' + QUOTENAME(OBJECT_NAME(fkc.referenced_object_id)) AS [ref_obj]
        ,STUFF((
                SELECT ',' + QUOTENAME(COL_NAME(fcR.referenced_object_id, fcR.referenced_column_id))
                FROM sys.foreign_key_columns AS fcR
                WHERE fcR.constraint_object_id = fk.object_id
                FOR XML path('')
                ), 1, 1, '') AS [ref_col_csv]
    FROM sys.foreign_key_columns AS fkc
    INNER JOIN sys.foreign_keys AS fk ON fk.object_id = fkc.constraint_object_id
    INNER JOIN sys.objects AS oParent ON oParent.object_id = fkc.parent_object_id
    INNER JOIN sys.schemas AS schParent ON schParent.schema_id = oParent.schema_id
    INNER JOIN sys.objects AS oRef ON oRef.object_id = fkc.referenced_object_id
    INNER JOIN sys.schemas AS schRef ON schRef.schema_id = oRef.schema_id
    GROUP BY fkc.parent_object_id
        ,fkc.referenced_object_id
        ,fk.NAME
        ,fk.object_id
        ,schParent.NAME
        ,schRef.NAME
    ) AS const
ORDER BY const.const_name

下面是生成“DROP CONSTRAINT”语句的脚本:

PRINT N'-- DROP FOREIGN KEY CONSTRAINTS --';

SET NOCOUNT ON;

SELECT '
PRINT N''Dropping ' + fk.NAME + '...''
GO
ALTER TABLE [' + sch.NAME + '].[' + OBJECT_NAME(fk.parent_object_id) + ']' + ' DROP  CONSTRAINT ' + '[' + fk.NAME + ']
GO'
FROM sys.foreign_keys AS fk
INNER JOIN sys.schemas AS sch ON sch.schema_id = fk.schema_id
ORDER BY fk.NAME

(摘自http://www.sqljunkies.com/WebLog/roman/archive/2005/01/30/7037.aspx， 现在存档在时光倒流机中)

Foreign key constraints and check constraint are very useful for enforcing data integrity and business rules. There are certain scenarios though where it is useful to temporarily turn them off because their behavior is either not needed or could do more harm than good. I sometimes disable constraint checking on tables during data loads from external sources or when I need to script a table drop/recreate with reloading the data back into the table. I usually do it in scenarios where I don't want a time consuming process to fail because one or a few of many million rows have bad data in it. But I always turn the constraints back on once the process is finished and also in some cases I run data integrity checks on the imported data.

如果禁用外键约束，则可以插入父表中不存在的值。如果禁用检查约束，则可以将值放入列中，就像检查约束不存在一样。下面是禁用和启用表约束的一些例子:

——禁用所有表约束 修改表MyTable NOCHECK约束 ——启用所有表约束 修改MyTable，检查检查约束全部 ——禁用单个约束 修改表MyTable NOCHECK约束 ——启用单一约束 ALTER TABLE MyTable WITH CHECK CHECK CONSTRAINT MyConstraint