[英]Mysql: Create a view showing only a subset of a one-to-many relationship
假设我有一对多的关系,例如歌手桌,每个歌手都有演唱的歌曲数:DROP TABLE歌手; 创建表歌手(id bigint不为null的auto_increment,name varchar(255)不为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");
假设按时间顺序正确写出了歌曲行,例如,我可以找到Joe在查询中演唱的最新歌曲
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,其中包含每位歌手的一行以及该歌手演唱的最新歌曲。 换句话说,看起来像这样的表:
singer_id singer_name song_id song_name
1 Joe 3 baz
2 Bob 4 quux
似乎这需要以某种方式将上面的ORDER BY / LIMIT子句集成到视图构造/连接逻辑中-但我不知道如何实现。 这可能吗?
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)
我宁愿使用GROUP BY进行此类操作,因此这是我的解决方案:
(为了方便起见,我提出了意见,但您不必这样做)
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;
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