declare num number;   
begin   
    --从系统表中查询表是否存在唯一约束
    select count(1) into num from user_constraints t where t.table_name = upper('TABLENAME') and t.constraint_type = 'U' and t.constraint_name = 'CONSTRAINTNAME';       
    --如果不存在，使用快速执行语句添加唯一约束
    if num = 0 then   
       execute immediate  
       'alter table TABLENAME add constraint CONSTRAINTNAME unique (COLUMNNAME) using index tablespace TABLESPACE';  
    end if;  
end;  
/

declare num number;  
begin   
    --从系统表中查询表是否存在唯一约束  
    select count(1) into num from user_constraints t where t.table_name = upper('TABLENAME') and t.constraint_type = 'U' and t.constraint_name = 'CONSTRAINTNAME';       
    --如果存在，使用快速执行语句删除唯一约束  
    if num = 1 then   
       execute immediate  
       'alter table TABLENAME drop constraint CONSTRAINTNAME';  
    end if;  
end;  
/