如图所示为原表格结构。
使用： 1 as 'col_num','1' as 'col_char'
分别添加了1个数值列，一个字符列，即虚拟列。
select sno as 学号,sname as 姓名,class as 班级,"" as sex from student
sno as 学号,sname as 姓名,class as 班集 ,'' 性别 from student
