我想为此查询返回的每个string_agg元素强制添加新列(即Fiction,Mystery改为一列为“ Fiction”,下一列为“ Mystery”),并且2.)我需要能够将标签栏最多扩展到五个标签:
SELECT books.isbn_13 as "ISBN", title as "Title",
author as "Author",
string_agg(tag_name, ', ') as "Tags"
FROM books
LEFT JOIN book_tags on books.isbn_13 = book_tags.isbn_13
GROUP BY books.isbn_13;
现在一切看起来都不错,除了我希望每个Tag都用一列而不是逗号分隔的值。这是我当前的结果:
ISBN | Title | Author | Tags
1111111111111 | The Adventures of Steve | Russell Barron | Fiction, Mystery
2222222222222 | It's all a mystery to me | Mystery Man | Mystery
3333333333333 | Biography of a Programmer | Solo Artist | Biography
4444444444444 | Steve and Russel go to Mars | Russell Groupon |
6666666666666 | Newest Book you Must Have | Newbie Onthescene |
所需的结果(将标签分成多个的列):
ISBN | Title | Author | Tag1 | Tag2 | Tag3 | Tag4
1111111111111 | The Adventures of Steve | Russell Barron | Fiction | Mystery | Male Protagonists | Fantasy|
2222222222222 | It's all a mystery to me | Mystery Man | Mystery
3333333333333 | Biography of a Programmer | Solo Artist | Biography
4444444444444 | Steve and Russel go to Mars | Russell Groupon |
6666666666666 | Newest Book you Must Have | Newbie Onthescene |
SCHEMA for books table (parent):
CREATE TABLE public.books
(
isbn_13 character varying(13) COLLATE pg_catalog."default" NOT NULL,
title character varying(100) COLLATE pg_catalog."default",
author character varying(80) COLLATE pg_catalog."default",
publish_date date,
price numeric(6,2),
content bytea,
CONSTRAINT books_pkey PRIMARY KEY (isbn_13)
)
SCHEMA book_tags table:
CREATE TABLE public.book_tags
(
isbn_13 character varying(13) COLLATE pg_catalog."default" NOT NULL,
tag_name character varying(30) COLLATE pg_catalog."default" NOT NULL,
CONSTRAINT book_tags_pkey PRIMARY KEY (isbn_13, tag_name),
CONSTRAINT book_tags_isbn_13_fkey FOREIGN KEY (isbn_13)
REFERENCES public.books (isbn_13) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE CASCADE
)
我已经对分组依据进行了研究,但交叉表/数据透视表却没有运气。这似乎应该是一件简单的事情,但是我是一个初学者,还没有找到答案。预先感谢您的指导。
答案 0 :(得分:1)
With CTE as (
SELECT books.isbn_13 as "ISBN",
title as "Title",
author as "Author",
tag_name as "Tag",
row_number() over (partition by books.isbn_13) as rn
FROM books
LEFT JOIN book_tags
on books.isbn_13 = book_tags.isbn_13
)
SELECT "ISBN", "Title", "Author",
MAX( CASE WHEN rn = 1 THEN Tag END) as Tag1,
MAX( CASE WHEN rn = 2 THEN Tag END) as Tag2,
MAX( CASE WHEN rn = 3 THEN Tag END) as Tag3,
MAX( CASE WHEN rn = 4 THEN Tag END) as Tag4,
MAX( CASE WHEN rn = 5 THEN Tag END) as Tag5
FROM CTE
GROUP BY "ISBN", "Title", "Author";