学习了SQL的一些基础内容之后，今天开始学习SQL的函数，继续加油
SQL 拥有很多可用于计数和计算的内建函数：

1. Aggregate 函数：计算从列中取得的值，返回一个单一的值
   有用一些 Aggregate 函数：

• AVG() - 返回平均值
  select avg(menu_id) from hscon.uam_menus where menu_name like '%黑名单%'

• COUNT() - 返回行数
  select count(*) from hscon.sys_dict where dict_name like '%黑名单%'

• FIRST() - 返回第一个记录的值
  select * from hscon.uam_menus where ROWNUM <=1'

• LAST() - 返回最后一个记录的值
  select * from (select * from hsics.abcde1234 order by hsics.abcde1234.age DESC) where ROWNUM <=1'

• MAX() - 返回最大值
  select MAX(age) from hsics.abcde1234 order by hsics.abcde1234.age'
  max能获取到单独的某一个属性的值，但是如果想要获取整个第一行的值的话就可以使用【FIRST() - 返回第一个记录的值】这里的方法了，最大值的话可以用LAST的方法，就看自己灵活变通了

• MIN() - 返回最小值
  select MIN(age) from hsics.abcde1234 order by hsics.abcde1234.age'

• SUM() - 返回总和
  select SUM(age) from hsics.abcde1234 order by hsics.abcde1234.age'

• GROUP BY 用于结合聚合函数，根据一个或多个列对结果集进行分组
  select hsics.abcde1234.name,sum(age) as totalAge from hsics.abcde1234 group by hsics.abcde1234.name'
  HAVING 子句可以让我们筛选分组后的各组数据
  GROUP BY 实现多表连接
  select hsics.abcde1234.name,count(hsics.abcde.name) as nameShows from hsics.abcde left join hsics.abcde1234 on hsics.abcde1234.source_flag = hsics.abcde.source_flag group by hsics.abcde1234.name'
  

• HAVING 在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与聚合函数一起使用。
  select hsics.abcde1234.name,count(hsics.abcde.name) as nameShows from hsics.abcde left join hsics.abcde1234 on hsics.abcde1234.source_flag = hsics.abcde.source_flag group by hsics.abcde1234.name having count(hsics.abcde.name) > 0