How to join only one row in joined table with postgres?

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清歌不尽
清歌不尽 2021-01-31 01:31

I have the following schema:

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


        
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7条回答
  • 2021-01-31 02:06

    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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  • 2021-01-31 02:08

    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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  • 2021-01-31 02:18

    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

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  • 2021-01-31 02:24
    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
    
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  • 2021-01-31 02:27

    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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  • 2021-01-31 02:27
    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
    
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