在数据表设计时，UNIQUE与PRIMARY KEY两者的区别是（）A．UNIQUE要求数据表中至少存在一行数据，必须在建表后创建，而PRIMARY KEY无该限制B．UNIQUE允许空值存在，而PRIMARY KEY不允许C．创建UNIQUE后，该数据行允许被修改，而PRIMARY KEY则不允许D．UNIQUE列不能创建外键，而PRIMARY KEY可以创建外键
在数据表设计时，UNIQUE与PRIMARY KEY两者的区别是（）
A．UNIQUE要求数据表中至少存在一行数据，必须在建表后创建，而PRIMARY KEY无该限制
B．UNIQUE允许空值存在，而PRIMARY KEY不允许
C．创建UNIQUE后，该数据行允许被修改，而PRIMARY KEY则不允许
D．UNIQUE列不能创建外键，而PRIMARY KEY可以创建外键
参考答案和解析
UNIQUE允许空值存在，而PRIMARYKEY不允许
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