select distinct constraint_type from user_constraints; C - C P R U
似乎P表示主键，R表示外键，正确吗？什么是U和C？
从
12cR1 documentation：
C - Check constraint on a table P - Primary key U - Unique key R - Referential integrity V - With check option,on a view O - With read only,on a view H - Hash expression F - Constraint that involves a REF column S - Supplemental logging
