SQL中的开窗函数(窗口函数)

<窗口函数> over (partition by <用于分组的列名>
                order by <用于排序的列名> [rows between ?? and ???])

-- 如果我们想在每个班级内按成绩排名，得到下面的结果。
select *,
   rank() over (partition by 班级
                 order by 成绩 desc) as ranking
from 班级表;

select *,
   rank() over (order by 成绩 desc) as ranking,
   dense_rank() over (order by 成绩 desc) as dese_rank,
   row_number() over (order by 成绩 desc) as row_num
from 班级表

select *,
   sum(成绩) over (order by 学号) as current_sum,
   avg(成绩) over (order by 学号) as current_avg,
   count(成绩) over (order by 学号) as current_count,
   max(成绩) over (order by 学号) as current_max,
   min(成绩) over (order by 学号) as current_min
from 班级表

sum()/... over ([partition by 列名] [order by 列名] [rows between ... and ...] )
-- 从起点到当前行数据聚合
between unbounded preceding and current row 
-- 往前2行到往后1行的数据聚合
between 2 preceding and 1 following 

select name,subject,score,
sum(score) over() as sum1,
sum(score) over(partition by subject) as sum2,
sum(score) over(partition by subject order by score) as sum3, 
-- 由起点到当前行的窗口聚合，和sum3一样
sum(score) over(partition by subject order by score rows between unbounded preceding and current row) as sum4, 
-- 当前行和前面一行的窗口聚合
sum(score) over(partition by subject order by score rows between 1 preceding and current row) as sum5,
-- 当前行的前面一行和后面一行的窗口聚合
sum(score) over(partition by subject order by score rows between 1 preceding AND 1 following) as sum6,
-- 当前和后面所有的行
sum(score) over(partition by subject order by score rows between current row and unbounded following) as sum7
from t_fraction;
+-------+----------+--------+-------+-------+-------+-------+-------+-------+-------+
| name  | subject  | score  | sum1  | sum2  | sum3  | sum4  | sum5  | sum6  | sum7  |
+-------+----------+--------+-------+-------+-------+-------+-------+-------+-------+
| 孙悟空   | 数学       | 12     | 359   | 185   | 12    | 12    | 12    | 31    | 185   |
| 沙悟净   | 数学       | 19     | 359   | 185   | 31    | 31    | 31    | 104   | 173   |
| 猪八戒   | 数学       | 73     | 359   | 185   | 104   | 104   | 92    | 173   | 154   |
| 唐玄奘   | 数学       | 81     | 359   | 185   | 185   | 185   | 154   | 154   | 81    |
| 猪八戒   | 英语       | 11     | 359   | 80    | 11    | 11    | 11    | 26    | 80    |
| 孙悟空   | 英语       | 15     | 359   | 80    | 26    | 26    | 26    | 49    | 69    |
| 唐玄奘   | 英语       | 23     | 359   | 80    | 49    | 49    | 38    | 69    | 54    |
| 沙悟净   | 英语       | 31     | 359   | 80    | 80    | 80    | 54    | 54    | 31    |
| 孙悟空   | 语文       | 10     | 359   | 94    | 10    | 10    | 10    | 31    | 94    |
| 唐玄奘   | 语文       | 21     | 359   | 94    | 31    | 31    | 31    | 53    | 84    |
| 沙悟净   | 语文       | 22     | 359   | 94    | 53    | 53    | 43    | 84    | 63    |
| 猪八戒   | 语文       | 41     | 359   | 94    | 94    | 94    | 63    | 63    | 41    |
+-------+----------+--------+-------+-------+-------+-------+-------+-------+-------+