Question

说我有一对多的关系，比如一个歌手表，每个人都唱过多首歌： DROP TABLE歌手; CREATE TABLE歌手（ id bigint not null auto_increment， name varchar（255）not null， PRIMARY KEY（id））;

INSERT INTO singers (name) VALUES ('Joe'), ('Bob');

DROP TABLE songs;
CREATE TABLE songs (
    id bigint not null auto_increment,
    singer_id bigint not null,
    name varchar(255) not null,
    PRIMARY KEY (id)
);

INSERT INTO songs (singer_id, name) VALUES (1, "foo"), (2, "bar"), (1, "baz"), (2, "quux");

假设歌曲行按时间顺序正确写出，例如我可以找到Joe与查询一起唱的最新歌曲

SELECT * FROM songs WHERE singer_id = 1 ORDER BY id DESC LIMIT 1;

现在，假设我想创建一个MySQL视图，其中包含每个歌手的一行以及该歌手演唱的最新歌曲。换句话说，一个看起来像这样的表：

singer_id   singer_name    song_id   song_name
   1          Joe             3         baz
   2          Bob             4         quux

似乎这需要将上面的ORDER BY / LIMIT子句以某种方式集成到视图构造/连接逻辑中 - 但我无法弄清楚如何。这可能吗？

Answer 1

select si.id, si.name, so.name, so.id 
from singers si
inner join songs so on so.singer_id = si.id
where so.id = (select max(songs.id) from songs where songs.singer_id=si.id)

http://sqlfiddle.com/#!2/d52c4/21

Answer 2

我宁愿使用GROUP BY来做这种事情，所以这是我的解决方案：

（我为了方便而发表意见，但你不必这样做）

CREATE VIEW lastsongs AS 
SELECT singers.id, singers.name, MAX(songs.id) AS lastsong
FROM singers
JOIN songs ON songs.singer_id = singers.id
GROUP BY singers.id, singers.name;

CREATE VIEW lastsongnames AS
SELECT lastsongs.name, songs.name AS songname
FROM lastsongs
JOIN songs ON songs.id = lastsongs.lastsong;

SELECT name, songname FROM lastsongnames;

Mysql：创建一个只显示一对多关系子集的视图

2 个答案: