我正在尝试以下想法。我正在尝试在单个查询中构建一个后备系统。以下是一些背景信息:
posts_table
id, timestamp
posts_content_table
id, post_id, language_id, title, description
这里的目标是查询 posts_table,然后在同一查询中,使用后备系统加入 posts_content_table。因此,如果我搜索不存在的 language_id 的帖子内容,它应该能够回退到包含内容的 language_id。它应该允许多个级别的后备。
示例:
SELECT p.id FROM posts_table AS p
LEFT OUTER JOIN posts_content_table AS pc ON (p.id=pc.post_id AND pc.language_id=1)
LEFT OUTER JOIN posts_content_table AS pc2 ON (p.id=pc2.post_id AND pc2.language_id=2)
LEFT OUTER JOIN posts_content_table AS pc3 ON (p.id=pc3.post_id AND pc3.language_id=3)
自然地,当它返回的数据不为空时,该数据应该被用作最终结果。这让我想到了案例。我将如何实现案例以这种方式工作?以下内容就足够了吗?
SELECT p.id, CASE title
WHEN pc.title is Null THEN pc2.title
WHEN pc2.title is Null THEN ps3.title
WHEN pc3.title is Null THEN '-'
END
FROM posts_table AS p
LEFT OUTER JOIN posts_content_table AS pc ON (p.id=pc.post_id AND pc.language_id=1)
LEFT OUTER JOIN posts_content_table AS pc ON (p.id=pc.post_id AND pc.language_id=2)
LEFT OUTER JOIN posts_content_table AS pc ON (p.id=pc.post_id AND pc.language_id=3)
最佳答案
这就是COALESCE()
的目的,它返回其参数的第一个非空:
SELECT
p.id,
COALESCE(pc.title, pc2.title, pc3.title, '-') AS title
FROM posts_table AS p
LEFT OUTER JOIN posts_content_table AS pc ON (p.id=pc.post_id AND pc.language_id=1)
LEFT OUTER JOIN posts_content_table AS pc2 ON (p.id=pc2.post_id AND pc2.language_id=2)
LEFT OUTER JOIN posts_content_table AS pc3 ON (p.id=pc3.post_id AND pc3.language_id=3)
(注意:我修复了您的表别名 - 它们都是 pc
,我假设您打算将别名设置为 pc、pc2、pc3
)
关于具有多个左外连接的 Mysql 案例,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/11413641/