Question

我想从三个表中提取数据，但如果第三个表中不存在数据，则应拉出其他两个表中的数据。

我现在有一个查询：

SELECT m.name, m.zipcode, p.lat, p.lon, meta.meta_value
FROM members AS m
INNER JOIN zipcodes AS p ON m.zipcode = p.zipcode
INNER JOIN usermeta AS meta ON m.id = meta.id
WHERE m.zipcode = p.zipcode AND meta.meta_key = 'image' m.country = "USA"

所有人都不存在

m.id和meta.id。

对于那些m.id的人，“图像”并不存在。

我想从其他2个表中提取数据，与“image”和m.id的存在无关。

我该怎么做？

Answer 1

您正在使用INNER JOIN;使用OUTER JOINS从条件为false的连接表中的列中获取NULL值。

例如：

SELECT m.name, m.zipcode, p.lat, p.lon, meta.meta_value
FROM members AS m
LEFT OUTER JOIN zipcodes AS p ON m.zipcode = p.zipcode
LEFT OUTER JOIN usermeta AS meta ON m.id = meta.id
WHERE m.zipcode = p.zipcode AND meta.meta_key = 'image' m.country = "USA"

Answer 2

如何将查询更改为

SELECT m.name, m.zipcode, p.lat, p.lon, meta.meta_value
FROM members AS m
INNER JOIN zipcodes AS p ON m.zipcode = p.zipcode 
LEFT OUTER JOIN usermeta AS meta ON m.id = meta.id
WHERE m.zipcode = p.zipcode AND COALESCE(meta.meta_key,'image')='image' AND m.country = 'USA'

来自三个表的数据

2 个答案: