如果 MySQL 查询中存在则加入答案

【问题标题】：Join If Exists in a MySQL query如果 MySQL 查询中存在则加入
【发布时间】：2011-09-10 21:34:30
【问题描述】：

我正在运行一份报告，将委员会成员的信息导出到 Excel 电子表格中。

这是我的查询：

SELECT membership_organization.name AS Firm, 
membership_individual.first AS FirstName, 
membership_individual.middle AS MiddleName, 
membership_individual.last AS LastName, 
membership_individual.email AS Email, 
membership_individual.phone AS Phone, 
membership_location.addr1 AS Address1, 
membership_location.addr2 AS Address2, 
membership_location.city AS City, 
membership_location.state AS State, 
membership_location.zipcode AS Zip 
FROM membership_individual 
JOIN membership_organization ON membership_individual.org_name_id = membership_organization.id 
JOIN membership_location ON membership_individual.location_id = membership_location.id 
WHERE membership_individual.id IN ({list if ids}) 
ORDER BY LastName

问题是一些成员没有设置位置 ID，或者它设置为 0，因此这些成员不会显示在报告中。

有什么方法可以让我限定位置 JOIN？如果成员位置 id 存在，请提取信息，如果不显示可用信息。

【问题讨论】：

标签： mysql sql join

【解决方案1】：

使用左连接，意味着你的查询将变成

SELECT membership_organization.name AS Firm, 
membership_individual.first AS FirstName, 
membership_individual.middle AS MiddleName, 
membership_individual.last AS LastName, 
membership_individual.email AS Email, 
membership_individual.phone AS Phone, 
membership_location.addr1 AS Address1, 
membership_location.addr2 AS Address2, 
membership_location.city AS City, 
membership_location.state AS State, 
membership_location.zipcode AS Zip 
FROM membership_individual 
JOIN membership_organization ON membership_individual.org_name_id = membership_organization.id 
LEFT JOIN membership_location ON membership_individual.location_id = membership_location.id 
WHERE membership_individual.id IN ({list if ids}) 
ORDER BY LastName

您将获得所有成员的数据，即使是位置表中没有行的成员。

【讨论】：

【解决方案2】：

尝试使用

LEFT JOIN membership_location ON membership_individual.location_id = membership_location.id

而不是你的变种..

【讨论】：

【解决方案3】：

使用左外连接：

SELECT membership_organization.name AS Firm, 
membership_individual.first AS FirstName, 
membership_individual.middle AS MiddleName, 
membership_individual.last AS LastName, 
membership_individual.email AS Email, 
membership_individual.phone AS Phone, 
membership_location.addr1 AS Address1, 
membership_location.addr2 AS Address2, 
membership_location.city AS City, 
membership_location.state AS State, 
membership_location.zipcode AS Zip 
FROM membership_individual 
JOIN membership_organization ON membership_individual.org_name_id = membership_organization.id 
LEFT OUTER JOIN membership_location ON membership_individual.location_id = membership_location.id 
WHERE membership_individual.id IN ({list if ids}) 
ORDER BY LastName

【讨论】：

outer 是隐含的。 left join 就足够了。

【解决方案4】：

将您的两个JOINs 更改为LEFT JOINs。您将从membership_individual 中获取where 子句匹配的所有记录，以及其他行不匹配的表的NULL 值。

【讨论】：

是的，我认为这很简单，我想我需要喝杯咖啡。谢谢！