pg 窗口函数

执行顺序

1. 选表
- FROM
2. 联表
- JOIN ON
3. 原始行过滤
- WHERE
4. 分组
- GROUP BY
5. 组级过滤
- HAVING
6. 投影及分析计算
- SELECT
	- DISTINCT
	- OVER
		- PARTITION BY
		- ORDER BY
		- ROWS/RANGE/GROUPS BETWEEN AND

7. 排序与结果裁剪
- ORDER BY
- LIMIT
- OFFSET

窗口帧（Window Frame）关键字

ROWS | RANGE | GROUPS
BETWEEN <frame_start> AND <frame_end>

样例

DROP TABLE IF EXISTS student;

CREATE TABLE student (
    sid INT PRIMARY KEY,        -- 学号
    name VARCHAR(50),           -- 学生姓名
    class_id INT,               -- 班级编号
    score INT                   -- 分数
);

INSERT INTO student (sid, name, class_id, score) VALUES
-- A班
(1, 'Alice', 101, 85),
(2, 'Bob', 101, 90),
(3, 'Charlie', 101, 78),
(4, 'David', 101, 90),

-- B班
(5, 'Eve', 102, 88),
(6, 'Frank', 102, 76),
(7, 'Grace', 102, 95),
(8, 'Heidi', 102, 84),

-- C班
(9, 'Ivan', 103, 70),
(10, 'Judy', 103, 82),
(11, 'Mallory', 103, 78),
(12, 'Niaj', 103, 85),

-- D班
(13, 'Olivia', 104, 91),
(14, 'Peggy', 104, 87),
(15, 'Sybil', 104, 79),
(16, 'Trent', 104, 93);

SELECT
  sid,
  class_id,
  score,
  AVG(score) OVER (PARTITION BY class_id) AS avg_score, -- 班级平均分
  MAX(score) OVER (PARTITION BY class_id) AS max_score, -- 班级最高分
  MIN(score) OVER (PARTITION BY class_id) AS min_score -- 班级最低分
FROM
  student;
  
SELECT
  sid,
  class_id,
  score,
  RANK() OVER (PARTITION BY class_id ORDER BY score DESC) AS rank_in_class, -- 排名（有并列，有空缺）
  DENSE_RANK() OVER (PARTITION BY class_id ORDER BY score DESC) AS DENSE_RANK, -- 排名（有并列，无空缺）
  ROW_NUMBER() OVER (PARTITION BY class_id ORDER BY score DESC) AS row_num -- 排名 （无并列，无空缺）
FROM
  student;
  
SELECT
  sid,
  class_id,
  score,
  SUM(score) OVER (PARTITION BY class_id ORDER BY score ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cumulative_score
FROM
  student;
  
SELECT
  sid,
  score,
  class_id,
  SUM(score) OVER (PARTITION BY class_id ORDER BY score RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS range_sum
FROM
  student;
  
SELECT
  sid,
  score,
  class_id,
  SUM(score) OVER (PARTITION BY class_id ORDER BY score GROUPS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS range_sum
FROM
  student;
  
SELECT
  sid,
  class_id,
  score,
  LAG(score) OVER (PARTITION BY class_id ORDER BY score) AS prev_score, -- 上一行分数
  LEAD(score) OVER (PARTITION BY class_id ORDER BY score) AS next_score -- 下一行分数
FROM
  student;
  
SELECT
  sid,
  class_id,
  score,
  AVG(score) OVER (PARTITION BY class_id) AS avg_score,
  RANK() OVER (PARTITION BY class_id ORDER BY score DESC) AS rank_in_class,
  CASE
    WHEN score > AVG(score) OVER (PARTITION BY class_id) THEN
      'above_avg'
    ELSE
      'below_avg'
  END AS performance
FROM
  student;