方便函數
===
## 知識點
- length #長度
- concat # 連接兩個值
- as # 別名
- substring # 切分字符串 string(字符串,開頭,結尾)
- random #隨機數
## 實戰
```
> select player,length(player) from users;
> select playyer,concat(player,'/',team) from users;
> select player,concat(player,'/',team) as "球員信息" from users;
> select substring(team,1,1) as "球隊首文字" from users;
> select concat("我",substring(team,1,1) ) as "球隊首文字" from users;
> select random();
> select * from users order by random();
> select * from users order by random() limit 1;
```
