pg 窗口函数

窗口函数基于结果集进行计算，将计算出的结果合并到输出的结果集上，并返回多行。使用窗口函数能大幅度简化SQL代码。
gaussdb提供内置的窗口函数，例如row_num()、rank()、lag()等，除了内置的窗口函数外，聚合函数、自定义函数后接OVER属性也可以作为窗口函数。
1，创建测试表并插入数据。

postgres=# DROP TABLE IF EXISTS scores;
NOTICE:  table "scores" does not exist, skipping
DROP TABLE
postgres=# CREATE TABLE scores(id serial PRIMARY KEY,subject varchar(32),stu_name varchar(32),score numeric(3,0));
CREATE TABLE
postgres=# INSERT INTO scores(subject,stu_name,score) VALUES('Chinese','user1',80),('Chinese','user2',90),('Chinese','user3',90),('math','user1',90),('math','user2',80),('math','user3',100),('English','user1',80),('English','user2',90),('English','user3',70);
INSERT 0 9
postgres=# SELECT * FROM scores;
 id | subject | stu_name | score
----+---------+----------+-------
  1 | Chinese | user1    |    80
  2 | Chinese | user2    |    90
  3 | Chinese | user3    |    90
  4 | math    | user1    |    90
  5 | math    | user2    |    80
  6 | math    | user3    |   100
  7 | English | user1    |    80
  8 | English | user2    |    90
  9 | English | user3    |    70
(9 rows)

2，avg() OVER()计算分组后数据的平均值。

postgres=# SELECT subject,stu_name,score,avg(score) OVER(PARTITION BY subject) FROM scores;
 subject | stu_name | score |         avg
---------+----------+-------+---------------------
 Chinese | user1    |    80 | 86.6666666666666667
 Chinese | user2    |    90 | 86.6666666666666667
 Chinese | user3    |    90 | 86.6666666666666667
 English | user3    |    70 | 80.0000000000000000
 English | user1    |    80 | 80.0000000000000000
 English | user2    |    90 | 80.0000000000000000
 math    | user1    |    90 | 90.0000000000000000
 math    | user2    |    80 | 90.0000000000000000
 math    | user3    |   100 | 90.0000000000000000
(9 rows)

3，row_number() OVER()对分组后的数据标注行号，从1开始。

postgres=# SELECT row_number() OVER(PARTITION BY subject ORDER BY score DESC),* FROM scores;
 row_number | id | subject | stu_name | score
------------+----+---------+----------+-------
          1 |  2 | Chinese | user2    |    90
          2 |  3 | Chinese | user3    |    90
          3 |  1 | Chinese | user1    |    80
          1 |  8 | English | user2    |    90
          2 |  7 | English | user1    |    80
          3 |  9 | English | user3    |    70
          1 |  6 | math    | user3    |   100
          2 |  4 | math    | user1    |    90
          3 |  5 | math    | user2    |    80
(9 rows)

4，rank() OVER()与row_number() OVER()类似主要区别是当组内某行字段值相同时，行号重复并且行号产生间隙。

postgres=# SELECT rank() OVER(PARTITION BY subject ORDER BY score DESC),* FROM scores;
 rank | id | subject | stu_name | score
------+----+---------+----------+-------
    1 |  2 | Chinese | user2    |    90
    1 |  3 | Chinese | user3    |    90
    3 |  1 | Chinese | user1    |    80
    1 |  8 | English | user2    |    90
    2 |  7 | English | user1    |    80
    3 |  9 | English | user3    |    70
    1 |  6 | math    | user3    |   100
    2 |  4 | math    | user1    |    90
    3 |  5 | math    | user2    |    80
(9 rows)

5，dense_rank() OVER()与rank() 类似，主要区别为当组内某行字段值相同时，虽然重复行号，但行号不产生间隙。

postgres=# SELECT dense_rank() OVER(PARTITION BY subject ORDER BY score DESC),* FROM scores;
 dense_rank | id | subject | stu_name | score
------------+----+---------+----------+-------
          1 |  2 | Chinese | user2    |    90
          1 |  3 | Chinese | user3    |    90
          2 |  1 | Chinese | user1    |    80
          1 |  8 | English | user2    |    90
          2 |  7 | English | user1    |    80
          3 |  9 | English | user3    |    70
          1 |  6 | math    | user3    |   100
          2 |  4 | math    | user1    |    90
          3 |  5 | math    | user2    |    80
(9 rows)

6，lag() OVER()可以获取行偏移offset那行字段的数据。

postgres=# SELECT LAG(id,-1) OVER(),* FROM scores;
 lag | id | subject | stu_name | score
-----+----+---------+----------+-------
   2 |  1 | Chinese | user1    |    80
   3 |  2 | Chinese | user2    |    90
   4 |  3 | Chinese | user3    |    90
   5 |  4 | math    | user1    |    90
   6 |  5 | math    | user2    |    80
   7 |  6 | math    | user3    |   100
   8 |  7 | English | user1    |    80
   9 |  8 | English | user2    |    90
     |  9 | English | user3    |    70
(9 rows)

postgres=# SELECT LAG(id,1,100) OVER(),* FROM scores;--不存在时指定默认值
 lag | id | subject | stu_name | score
-----+----+---------+----------+-------
 100 |  1 | Chinese | user1    |    80
   1 |  2 | Chinese | user2    |    90
   2 |  3 | Chinese | user3    |    90
   3 |  4 | math    | user1    |    90
   4 |  5 | math    | user2    |    80
   5 |  6 | math    | user3    |   100
   6 |  7 | English | user1    |    80
   7 |  8 | English | user2    |    90
   8 |  9 | English | user3    |    70
(9 rows)

7，first_value() OVER()用来取结果集每一个分组的第一行数据的字段值。

postgres=# SELECT first_value(score) OVER(PARTITION BY subject ORDER BY score DESC),* FROM scores;
 first_value | id | subject | stu_name | score
-------------+----+---------+----------+-------
          90 |  2 | Chinese | user2    |    90
          90 |  3 | Chinese | user3    |    90
          90 |  1 | Chinese | user1    |    80
          90 |  8 | English | user2    |    90
          90 |  7 | English | user1    |    80
          90 |  9 | English | user3    |    70
         100 |  6 | math    | user3    |   100
         100 |  4 | math    | user1    |    90
         100 |  5 | math    | user2    |    80
(9 rows)

8，last_value() OVER()用来取结果集每一个分组的最后一行数据的字段值。

postgres=# SELECT last_value(score) OVER(PARTITION BY subject),* FROM scores;
 last_value | id | subject | stu_name | score
------------+----+---------+----------+-------
         90 |  1 | Chinese | user1    |    80
         90 |  2 | Chinese | user2    |    90
         90 |  3 | Chinese | user3    |    90
         90 |  9 | English | user3    |    70
         90 |  7 | English | user1    |    80
         90 |  8 | English | user2    |    90
        100 |  4 | math    | user1    |    90
        100 |  5 | math    | user2    |    80
        100 |  6 | math    | user3    |   100
(9 rows)

9，nth_value() OVER()用来取结果集每一个分组的指定行数据的字段值。

postgres=# SELECT nth_value(score,2) OVER(PARTITION BY subject),* FROM scores;
 nth_value | id | subject | stu_name | score
-----------+----+---------+----------+-------
        90 |  1 | Chinese | user1    |    80
        90 |  2 | Chinese | user2    |    90
        90 |  3 | Chinese | user3    |    90
        80 |  9 | English | user3    |    70
        80 |  7 | English | user1    |    80
        80 |  8 | English | user2    |    90
        80 |  4 | math    | user1    |    90
        80 |  5 | math    | user2    |    80
        80 |  6 | math    | user3    |   100
(9 rows)

10，如果窗口函数需要多次使用，可以使用窗口函数别名。

postgres=# SELECT avg(score) OVER(r),sum(score) OVER(r),* FROM scores WINDOW r AS (PARTITION BY subject);
         avg         | sum | id | subject | stu_name | score
---------------------+-----+----+---------+----------+-------
 86.6666666666666667 | 260 |  1 | Chinese | user1    |    80
 86.6666666666666667 | 260 |  2 | Chinese | user2    |    90
 86.6666666666666667 | 260 |  3 | Chinese | user3    |    90
 80.0000000000000000 | 240 |  9 | English | user3    |    70
 80.0000000000000000 | 240 |  7 | English | user1    |    80
 80.0000000000000000 | 240 |  8 | English | user2    |    90
 90.0000000000000000 | 270 |  4 | math    | user1    |    90
 90.0000000000000000 | 270 |  5 | math    | user2    |    80
 90.0000000000000000 | 270 |  6 | math    | user3    |   100
(9 rows)