4.postgresql--rollup,grouping sets,cube

PostgreSQL ROLLUP 是group by 的子句，是生成多个分组集合的快捷功能。与Cube子句的差异是，rollup 不生成基于特定列所有可能的分组集合，生成分组集合为其子集。

ROLLUP假设输入列之间存在层次结构，从而生成有意义的所有分组集合。这就是为什么ROLLUP经常用于生成报表的小计和总计。

cube (c1,c2,c3)生成所有8中可能：

(c1, c2, c3)
(c1, c2)
(c2, c3)
(c1,c3)
(c1)
(c2)
(c3)
()

而ROLLUP(c1,c2,c3)仅生成4种分组集合，假设层级c1 > c2 > c3：

(c1, c2, c3)
(c1, c2)
(c1)
()

CREATE TABLE geeksforgeeks_courses(
    course_name VARCHAR NOT NULL,
    segment VARCHAR NOT NULL,
    quantity INT NOT NULL,
    PRIMARY KEY (course_name, segment)
);

INSERT INTO geeksforgeeks_courses(course_name, segment, quantity)
VALUES
    ('Data Structure in Python', 'Premium', 100),
    ('Algorithm Design in Python', 'Basic', 200),
    ('Data Structure in Java', 'Premium', 100),
    ('Algorithm Design in Java', 'Basic', 300);

   select  course_name,
    segment,
    SUM (quantity)
    from  geeksforgeeks_courses
    group by (course_name, segment) order by course_name,segment

 select  course_name,
    segment,
    SUM (quantity)
    from  geeksforgeeks_courses
    group by (course_name, segment) 
    union all 
    select  course_name,
    NULL,
    SUM (quantity)
    from  geeksforgeeks_courses
    group by (course_name) 
    
    union all 
     select  NULL,
    NULL,
    SUM (quantity)
    from  geeksforgeeks_courses
    order by course_name,segment
    

用如下的sql可以达到相同的效果

SELECT
    course_name,
    segment,
    SUM (quantity)
FROM
    geeksforgeeks_courses
GROUP BY
    ROLLUP (course_name, segment)
ORDER BY
    course_name,
    segment;

SELECT
    coalesce(course_name,'所有课程'),
    coalesce(segment,'所有'),
    SUM (quantity)
FROM
    geeksforgeeks_courses
GROUP BY
    ROLLUP (course_name, segment)
ORDER BY
    course_name,
    segment;

SELECT
    coalesce(course_name,'所有课程'),
    coalesce(segment,'所有'),
    SUM (quantity)
FROM
    geeksforgeeks_courses
GROUP BY
    cube (course_name, segment)

cube 分成了(), course_name,segment,course_name+segment
而rollup分成了(),course_name,course_name+segment

按照自定义进行分组

SELECT
    coalesce(course_name,'所有课程'),
    coalesce(segment,'所有'),
    SUM (quantity)
FROM
    geeksforgeeks_courses
GROUP by GROUPING SETS ((course_name),(segment),(course_name,segment),())