语义不匹配和不兼容数据格式是数据集成的主要问题来源，不太可能消失。
Semantic mismatches and incompatible data formats are a staple of data integration and are not likely to vanish.
保守式并发在更新期间锁定数据，使其他用户不会陷于提交不兼容数据更改的危险中。
Pessimistic concurrency locks data during update so that other users are not in danger of committing incompatible data changes.
对兼容数据类型的 NULL 值的处理是完全相同的；对不兼容数据类型的 NULL 值的处理可能不相同。
NULL values of compatible data types are treated identically; those of non-compatible data types may not be.
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