在已连接的表上连接表

Question

我正在努力整理一个连接 3 个表的 select 语句。

这是数据库：

CREATE TABLE Beerstyles
(
  style_id     INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  style_name   VARCHAR(50) NOT NULL
)ENGINE=InnoDB;

CREATE TABLE Breweries
(
  brew_id   INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  booth_num VARCHAR(10) NOT NULL,
  brew_name VARCHAR(50) NOT NULL
)ENGINE=InnoDB;

CREATE TABLE Beers
(
  beer_id      INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  beer_name    VARCHAR(50) NOT NULL,
  alc_vol      DECIMAL(2,1) NOT NULL,
  fk_style_id  INT NOT NULL,
  fk_brew_id   INT NOT NULL
)ENGINE=InnoDB;

CREATE TABLE Favorites
(
  fav_id      INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  user_id     VARCHAR(55) NOT NULL,
  fk_beer_id  INT NOT NULL,
  fav_comment VARCHAR(255)
)ENGINE=InnoDB;


ALTER TABLE Beers ADD CONSTRAINT FK_BeerStyle_Style FOREIGN KEY (fk_style_id) REFERENCES    Beerstyles (style_id);
ALTER TABLE Beers ADD CONSTRAINT FK_BeerBrew_Brew FOREIGN KEY (fk_brew_id) REFERENCES Breweries (brew_id);
ALTER TABLE Favorites ADD CONSTRAINT FK_FavBeer_Beer FOREIGN KEY (fk_beer_id) REFERENCES Beers (beer_id);

这是第一部分：

SELECT * FROM Favorites JOIN Beers ON Favorites.fk_beer_id = Beers.beer_id

我需要混合 brew_name，但无法正确设置。当我尝试加入 Breweries (ON Favorites.fk_brew_id = Breweries.brew_id) 时，我收到一条错误消息：“'on clause' 中的未知列 'Favorites.fk_brew_id'”

希望你们能帮帮我:)

Answer 1

Favorite 表中没有fk_brew_id，但您必须将JOIN 条件添加到Beers 表而不是Favorite 表中，如下所示：

SELECT 
  bw.brew_name,
  b.beer_name,
  f.fav_comment,
  ...
FROM Favorites       AS f
INNER JOIN Beers     AS b  ON f.fk_beer_id = b.beer_id
INNER JOIN Breweries AS bw ON b.fk_brew_id = bw.brew_id;

Answer 2

SELECT * 
FROM Favorites 
JOIN Beers 
   ON Favorites.fk_beer_id = Beers.beer_id
JOIN Breweries 
   ON Beers.fk_brew_id = Breweries.brew_id

当然在现实生活中你不应该使用 select *。当您进行连接时，您会重复列并导致查询速度变慢。