主键列

• The microsoft jet odbc driver always returns unique index and primary key columns as nullable regardless of whether they are nullable or not
  Microsoft jet odbc驱动程序总是将唯一的索引和主键列返回为可以为空，而不管它们是否可以为空。
• Because of the ability to have multiuser access to the primary key columns , it is important that the primary key column values not be changed during synchronizations
  由于可以让多个用户访问主键列，因此在同步期间不更改主键列值是很重要的。
• For example , if a tableadapter is configured to query data from a table that does not have a primary key column defined , it does not generate dbdirect methods
  例如，如果将tableadapter配置为从未定义主键列的表中查询数据，它将不会生成dbdirect方法。
• Requires extra processing at the data source to determine primary key column information . this additional processing can hinder performance
  当使用fillschema或将missingschemaaction设置为addwithkey时，将需要在数据源中进行额外的处理来确定主键列信息。
• If informational check constraints exist on the primary key column , you should explicitly specify informational check constraints on their corresponding foreign key columns
  如果在主键列上存在信息检查约束，那么应该在它们相应的外键列上指定信息检查约束。
• In a distributed environment it is essential to ensure that the same value is not inserted into a primary key column or any other unique column at more than one node
  在分布式环境中，确保没有将同一值插入到多个节点上的主键列或任何其他唯一列，这一点很重要。
• If no primary key columns are returned but unique columns are , the unique columns are used as the primary key if , and only if , all of the unique columns are nonnullable
  如果未返回主键列，但返回了唯一列，则当且仅当所有唯一列都不可为空时，将唯一列用作主键。
• If no primary key columns are returned but unique columns are , the unique columns are used as the primary key if , and only if , all the unique columns are nonnullable
  如果未返回任何主键列，但返回了唯一列，则当且仅当所有唯一列都不可为空时，唯一列才用作主键。