本文主要是介绍数据库中写聚合函数，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
数据库中聚合函数是指max，min，avg，sum，count
聚合函数对null值不计算
如果一行的数据都是null，计算count（*）则计算null的行数
sql语句的执行顺序，
1>form表
2>where条件
3>group by 列
4>Having 条件
5>select distinct top
6>order by
–查询tsID降序排列的前五个学生
select top 5 * from TblStudent order by tsID desc
–查询结果合并为一列
select top 5
tsAddress+’ ‘+tsName as studentName
from TblStudent order by tsID desc
–按百分比求，向上取整
select top 10 percent * from TblStudent order by tsID desc
select distinct
tsName,tsAddress,tsClassID
from TblStudent
–查询TblStudent中tsAge的平均数，作为平均年龄这一列，三种写法
select AVG(tsAge) as 平均年龄 from TblStudent
select AVG(tsAge) 平均年龄 from TblStudent
select 平均年龄=AVG(tsAge) from TblStudent
select sum(tsAge) as 总年龄 from TblStudent
–null也被计算在内
select count(tsAge) as 记录总数 from TblStudent
–一个语句执行多条查询
select
count(tsAge) as 记录总数,
sum(tsAge) as 总年龄,
AVG(tsAge) as 平均年龄
from TblStudent
–子查询在查询里面查询查询语句
select * from
(select AVG(tsAge) as 平均年龄 from TblStudent) as a,
(select AVG(tsAge) 平均年龄 from TblStudent) as b,
(select 平均年龄=AVG(tsAge) from TblStudent ) as c
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