当前SC表数据：
SNO CNO SCORE
s001 c001 78.90 s002 c001 80.90 s003 c001 81.90 s004 c001 60.90 s001 c002 82.90 s002 c002 72.90 s003 c002 81.90 s001 c003 59.00
1.给SC表中的SNO列名取别名为“学生号”：
方法1.select SNO 学生号 from sc;
方法2.select SNO as 学生号 from sc;
可见给列名取别名是as可有可无
查询结果为：
学生号
s001 s001 s001 s002 s002 s003 s003 s004
2.给SC表取别名为“成绩表”：
方法：select * from sc 成绩表; 或者 select a.* from sc 成绩表;
oracle中给表取别名不可以用as哦，select * from sc as 成绩表; 这种会报错
3.将SC表中的SNO列名取别名为学生号、CNO列名取别名为科目、SCORE列名取别名为成绩，并将SC表取别名为a表：
select a.SNO as 学生号，a.CNO as 课程，a.SCORE as 分数 from sc a;
查询结果为：
学生号 科目 分数
s001 c001 78.90 s002 c001 80.90 s003 c001 81.90 s004 c001 60.90 s001 c002 82.90 s002 c002 72.90 s003 c002 81.90 s001 c003 59.00
