函数
①转小写：lower()
示例：select lower(ename) from emp;
②转大写：upper()
示例：select * from emp where job=upper(\'manager\');
③截取字符串：substr(要截取的字段，开始位置（从1开始），要截取的长度)
示例：select substr(applabel,1,4) from appconfig;
④获取字段长度：length()
示例：select length(appid) from appconfig;
⑤去除首尾空格，不包含中间的空格
示例：select * from emp where job=trim(upper(\'manager \'));
⑥将字符串转为日期：str_to_date(字符串，匹配格式)
示例：select * from emp where HIREDATE=str_to_date(\'1981-02-20\',\'%Y-%m-%d\');
⑦获取当前日期：now()
⑧千分位：format(需要插入千分位的字段，位数)
示例：select empno, ename, Format(sal, 2) from emp;
⑨四舍五入：round()
示例：select round(123.56);
⑩生成随机数：rand()
示例：select rand();
①①case..when..then..else..then..end
示例：如果job为MANAGERG薪水上涨10%,如果job为SALESMAN工资上涨50%
select empno, ename, job, sal, case job when \'MANAGER\' then sal*1.1 when \'SALESMAN\' then sal*1.5 end as newsal from emp;
①②ifnull()
示例：select ifnull(comm,0) from emp;
如果comm为空，则设置为0