MySQL为null不返回任何内容

Question

我有以下查询：

select 
    emp.name, emp.birthdate, emp.ssn, emp.current_employee, 
    german.introduction as "German Intro", german.work_experience as "German Work Experience", german.education as "German Education",
    chinese.introduction as "Chinese Intro", chinese.work_experience as "Chinese Work Experience", chinese.education as "Chinese Education"
from employees emp
left join contact_info german on emp.id = german.id_employee
left join contact_info chinese on emp.id = chinese.id_employee
where emp.name like '%peter%' 
and (german.id_language = 1 or german.id_language IS NULL)
and (chinese.id_language = 2 or chinese.id_language IS NULL)

表的创建查询：

CREATE TABLE `contact_info` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `id_employee` int(11) NOT NULL,
  `id_language` int(11) NOT NULL,
  `introduction` text,
  `work_experience` text,
  `education` text,
  PRIMARY KEY (`id`)
) 
CREATE TABLE `employees` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL DEFAULT '',
  `birthdate` date NOT NULL,
  `ssn` varchar(20) NOT NULL DEFAULT '',
  `current_employee` enum('Y','N') NOT NULL DEFAULT 'Y',
  PRIMARY KEY (`id`)
)
CREATE TABLE `languages` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `language_name` varchar(255) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) 

我在员工日志中确实有Peter的记录。 在contact_info表中有一行包含德语文本，但在contact_info表中无一行包含中文文本

我希望获得有关Peter的所有信息，其中包含德语文本，而中文文本为NULL，但是最终没有结果。 知道我缺少什么吗？

额外信息：语言表包含3种语言：德语，中文和英语，将来可能会添加更多语言。 如果可以通过某种方式设置此查询，使其立即自动加载所有语言，那就太棒了

Answer 1

我认为您需要这样做，并将id_language添加到您的加入条件中。

select 
    emp.name, emp.birthdate, emp.ssn, emp.current_employee, 
    german.introduction as "German Intro", german.work_experience as "German Work Experience", german.education as "German Education",
    chinese.introduction as "Chinese Intro", chinese.work_experience as "Chinese Work Experience", chinese.education as "Chinese Education"
from employees emp
left join contact_info german on emp.id = german.id_employee AND german.id_language = 1
left join contact_info chinese on emp.id = chinese.id_employee AND chinese.id_language = 2
where emp.name like '%peter%' 
and (german.id_language = 1 or german.id_language IS NULL)
and (chinese.id_language = 2 or chinese.id_language IS NULL)

Answer 2

当您使用left join ，条件（通常）应on而不是where 。 所以：

select emp.name, emp.birthdate, emp.ssn, emp.current_employee, 
       german.introduction as "German Intro", german.work_experience as "German Work Experience", german.education as "German Education",
       chinese.introduction as "Chinese Intro", chinese.work_experience as "Chinese Work Experience", chinese.education as "Chinese Education"
from employees emp left join
     contact_info german 
     on emp.id = german.id_employee and german.id_language = 1 left join 
     contact_info chinese
     on emp.id = chinese.id_employee and chinese.id_language = 2
where emp.name like '%peter%' ;

在where包含条件是多余的。