查(select * from 表名)
基本语法：
select <字段1，字段2，...> from <表名> where <表达式>;
例如，查询student表中所有的记录，在终端窗口输入命令：
select id,name,age from employee;
也可以查询限定的记录，输入如下命令，可以限定查询结果为第0条到第1条记录，也就是返回第一条记录：
select * from empolyee limit 0,3;
也可以查询通过年龄限制查询
select * from employee where age between 18 and 20;
模糊查询(like)：
select * from employee where name like "阿%";
查询空值：
select name from employee where salary is null;
排序查询(order by)
select name,chengji from employee order by chengji;
select name,chengji from employee where chengji>50 order by chengji;
降序查询：
select name,chengji from employee where chengji>50 order by chengji desc;
as 可以添加一个字段：
select name,chengji1+chengji2 as 总成绩 from employee;
select name,chengji1+chengji2 as 总成绩 from employee order by 总成绩;
分组查询 group by :
注意：按分组条件分组每一组只会显示第一条记录；
select * from employee group by name;(按照name分组 name一样的将被分为一组)
