从约束的定义来看是不能重用的。Oracle Database enables you to apply constraints both at the table and column level. A constraint specified as part of the definition of a column or attribute is called an inline specification. A constraint specified as part of the table definition is called an out-of-line specification.http://download.oracle.com/docs/cd/E11882_01/server.112/e10713/datainte.htm#CNCPT322
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