第一章：学习SQL基础> alter user scott account unlock; SQL> alter user scott identified by tiger; SQL> conn scott tiger 1。使用算术操作符SELECT last name salary + 300 FROM employees; 7。使用interval创建表格warranty prod id number warranty time INTERVAL YEAR 3 TO MONTH ; INSERT INTO warranty VALUES 123 INTERVAL "8" MONTH ; INSERT INTO warranty VALUES 155 INTERVAL "200" YEAR 3 ; INSERT INTO warranty VALUES 678 "200 11" ; SELECT FROM warranty; 8。使用extract函数抽取SELECT EXTRACT YEAR FROM SYSDATE FROM DUAL; 9。使用FROM TZ转化时间戳SELECT FROM TZ TIMESTAMP "2000 03 28 08:00" "3:00" FROM DUAL;通过查看v$reserved words可以查看oracle的保留关键字drop table name purge;不经过回收站drop table name cascade constraints;删除父表的时候把子表也删除（外健） with check option该列有访问且值等（即条件一致） novalidate enable约束