方便函数
===
## 知识点
- length #长度
- concat # 连接两个值
- as # 别名
- substring # 切分字符串 string(字符串,开头,结尾)
- random #随机数
## 实战
```
> select player,length(player) from users;
> select playyer,concat(player,'/',team) from users;
> select player,concat(player,'/',team) as "球员信息" from users;
> select substring(team,1,1) as "球队首文字" from users;
> select concat("我",substring(team,1,1) ) as "球队首文字" from users;
> select random();
> select * from users order by random();
> select * from users order by random() limit 1;
```
