题目内容

长度在500米以上的隧道应设贯通整个隧道的救援通道.双线隧道设置在两侧。

A. 正确
B. 错误

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Which two statements are true about constraints? （）

A. The UNIQUE constraint does not permit a null value for the column.
B. A UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.
C. The PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.
D. The NOT NULL constraint ensures that null values are not permitted for the column.

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