数据库学习二

** 顺序
form ->where-> select-> order by

                                    语法
 1，||是将两个列组合起来。
 2，as或者空格是将列重新另名。
 例如：select last_name || job_id as "Employee" from employees;
 3,distinct是去除重合的,用法 select distinct 
 例如：select distinct department_id from employees;
 4,escape为转义符.用法'SA\_%' escape '\';