MySQL LEFT JOIN返回空结果集

Question

也许我想念一些愚蠢但是......
我有三张m-m关系表:

CREATE TABLE tbl_users (
  usr_id        INT NOT NULL AUTO_INCREMENT ,
  usr_name      VARCHAR( 64 ) NOT NULL DEFAULT '' ,
  usr_surname   VARCHAR( 64 ) NOT NULL DEFAULT '' ,
  usr_pwd       VARCHAR( 64 ) NOT NULL ,
  usr_level     INT( 1 ) NOT NULL DEFAULT 0,
  PRIMARY KEY ( usr_id )
) ENGINE = InnoDB;


CREATE TABLE tbl_houses (
  house_id      INT NOT NULL AUTO_INCREMENT ,
  city          VARCHAR( 100 ) DEFAULT '' ,
  address       VARCHAR( 100 ) DEFAULT '' ,
  PRIMARY KEY ( house_id )
) ENGINE = InnoDB;

CREATE TABLE tbl_users_houses (
  user_id       INT NOT NULL ,
  house_id      INT NOT NULL ,
  INDEX user_key (user_id),
  FOREIGN KEY (user_id) REFERENCES tbl_users(usr_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  INDEX house_key (house_id) ,
  FOREIGN KEY (house_id) REFERENCES tbl_houses(house_id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE = InnoDB;

进入链接表我有两个记录:

user_id  house_id
  1         1
  1         2

现在,尝试选择所有房屋:

select * from tbl_houses AS H 
  left join tbl_users_houses AS UH on H.house_id = UH.house_id
where UH.user_id = 2;

为什么我没有数据而不是所有房屋？

Answer 1

因为这条线:

where UH.user_id = 2;

仅当UH.user_id为非null时才会出现这种情况,因此它实际上排除了您在UH中没有匹配行的房屋的情况,这是使用LEFT JOIN的重点.

如果你想要所有的房屋,以及匹配的UH数据,请使用:

select * from tbl_houses AS H 
  left join tbl_users_houses AS UH on H.house_id = UH.house_id and UH.user_id = 2;