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`)
)

我在员工日志中有Peter的记录。我在contact_info表中有一行用德语文本，但我在contact_info表中没有中文文本的行

我希望得到关于彼得的所有信息，其中填充德语文本，中文名称为NULL，但最终没有结果。知道我错过了什么吗？

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。如果可以设置这个查询，它会自动加载所有语言，这将是非常棒的

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时，条件应该（通常）进入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中包含条件是多余的。

MySQL为null不返回任何内容

2 个答案: