如何在用 postgres 连接的表中只连接一行？

我有以下模式:

CREATE TABLE author (
id   integer
, name varchar(255)
);
CREATE TABLE book (
id        integer
, author_id integer
, title     varchar(255)
, rating    integer
);

我希望每个作者都能写出最后一本书:

SELECT book.id, author.id, author.name, book.title as last_book
FROM author
JOIN book book ON book.author_id = author.id


GROUP BY author.id
ORDER BY book.id ASC

显然你可以在 mysql: 在 MySQL 中联接两个表，只从第二个表返回一行中做到这一点。

但是 postgres 给出了这个错误:

ERROR: 列“ book.id”必须出现在 GROUP BY 子句中或者使用 在聚合函数中: SELECT book.id、 author.id、 author.name, Title as last _ book FROM author JOIN book book ON book.author _ id = Id GROUP BY auth.id ORDBY book.id ASC

这是因为:

当 GROUPBY 存在时，它对 SELECT 列表无效 表达式引用聚合以外的未分组列 函数，因为返回的值可能不止一个 用于未分组的列。

如何指定 postgres: “当按 joined_table.id排序时，只给我联接表中的最后一行?”

编辑: 根据这些数据:

INSERT INTO author (id, name) VALUES
(1, 'Bob')
, (2, 'David')
, (3, 'John');


INSERT INTO book (id, author_id, title, rating) VALUES
(1, 1, '1st book from bob', 5)
, (2, 1, '2nd book from bob', 6)
, (3, 1, '3rd book from bob', 7)
, (4, 2, '1st book from David', 6)
, (5, 2, '2nd book from David', 6);

我应该看看:

book_id author_id name    last_book
3       1         "Bob"   "3rd book from bob"
5       2         "David" "2nd book from David"
86283 次浏览
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Here is one way:

SELECT book_id, author_id, author_name, last_book
FROM (
SELECT b.id as book_id
, a.id as author_id
, a.name as author_name
, b.title as last_book
, row_number() over (partition by a.id
order by b.id desc) as rn
FROM author a
JOIN book b
ON b.author_id = a.id
) last_books
WHERE rn = 1;
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最佳答案 
select distinct on (author.id)
book.id, author.id, author.name, book.title as last_book
from
author
inner join
book on book.author_id = author.id
order by author.id, book.id desc

Check distinct on

SELECT DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. The DISTINCT ON expressions are interpreted using the same rules as for ORDER BY (see above). Note that the "first row" of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first.

With distinct on it is necessary to include the "distinct" columns in the order by. If that is not the order you want then you need to wrap the query and reorder

select
*
from (
select distinct on (author.id)
book.id, author.id, author.name, book.title as last_book
from
author
inner join
book on book.author_id = author.id
order by author.id, book.id desc
) authors_with_first_book
order by authors_with_first_book.name

Another solution is to use a window function as in Lennart's answer. And another very generic one is this

select
book.id, author.id, author.name, book.title as last_book
from
book
inner join
(
select author.id as author_id, max(book.id) as book_id
from
author
inner join
book on author.id = book.author_id
group by author.id
) s
on s.book_id = book.id
inner join
author on book.author_id = author.id
小开

This may look archaic and overly simple, but it does not depend on window functions, CTE's and aggregating subqueries. In most cases it is also the fastest.

SELECT bk.id, au.id, au.name, bk.title as last_book
FROM author au
JOIN book bk ON bk.author_id = au.id
WHERE NOT EXISTS (
SELECT *
FROM book nx
WHERE nx.author_id = bk.author_id
AND nx.book_id > bk.book_id
)
ORDER BY book.id ASC
;
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As a slight variation on @wildplasser's suggestion, which still works across implementations, you can use max rather than not exists. This reads better if you like short joins better than long where clauses

select *
from author au
join (
select max(id) as max_id, author_id
from book bk
group by author_id) as lb
on lb.author_id = au.id
join bk
on bk.id = lb.max_id;

or, to give a name to the subquery, which clarifies things, go with WITH

with last_book as
(select max(id) as max_id, author_id
from book bk
group by author_id)


select *
from author au
join last_book lb
on au.id = lb.author_id
join bk
on bk.id = lb.max_id;
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create temp table book_1 as (
SELECT
id
,title
,author_id
,row_number() OVER (PARTITION BY id) as rownum
FROM
book)  distributed by ( id );


select author.id,b.id, author.id, author.name, b.title as last_book
from
author


left  join
(select * from  book_1 where rownum = 1 ) b on b.author_id = author.id
order by author.id, b.id desc
小开

I've done something similar for a chat system, where room holds the metadata and list contains the messages. I ended up using the Postgresql LATERAL JOIN which worked like a charm.

SELECT MR.id AS room_id, MR.created_at AS room_created,
lastmess.content as lastmessage_content, lastmess.datetime as lastmessage_when
FROM message.room MR
LEFT JOIN LATERAL (
SELECT content, datetime
FROM message.list
WHERE room_id = MR.id
ORDER BY datetime DESC
LIMIT 1) lastmess ON true
ORDER BY lastmessage_when DESC NULLS LAST, MR.created_at DESC

For more info see https://heap.io/blog/engineering/postgresqls-powerful-new-join-type-lateral

小开

You could add a rule into the join for specifying only one row. I had work for me.

Like this:

SELECT
book.id,
author.id,
author.name,
book.title as last_book
FROM author auth1
JOIN book book ON (book.author_id = auth1.id AND book.id = (select max(b.id) from book b where b.author_id = auth1))
GROUP BY auth1.id
ORDER BY book.id ASC

This way you get the data from the book with the higher ID. You could add "date" and make the same with the max(date).


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