对于主键和外键，只有非空列才能被列出和选择来定义这些键。

For primary and unique keys, only the not null columns are listed and can be selected to define these keys.

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空英：有什么资格才能在美国居住或工作？

SC: What qualifies someone to live or work in U. S.?

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因此，我们在选股时必须特别注意主力的动向及其操作方法，才能精确判断其做空或做多的企图。

Therefore, we must pay special attention to stock movements and their main method of operation in order to accurately determine the short or attempt to do more.

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