Mysql 合并（字段“默认值”）不适用于postgresql+；轨道

上面是我对postgresql数据库的原始sql查询。我的问题是，如果数据库中的默认文件路径为空，我想显示它。为此，我尝试了三种方法：

sql = "SELECT DISTINCT(p.*), concat(u.first_name,' ',u.last_name) as fullname, concat('@',u.username) as username, cover.file_path, cover.file_type, avatar.avatar_path 
       FROM products p
       JOIN stores s ON s.id = p.store_id
       JOIN users u ON u.id = s.user_id
       LEFT OUTER JOIN (SELECT fh.product_id AS cp_id, 
       coalesce(fh.file_path, 'default image path') AS file_path,
       fh.file_type AS file_type FROM file_handlers fh WHERE is_cover = true) cover ON p.id = cover.cp_id
       LEFT OUTER JOIN (SELECT fh.id AS av_id, fh.file_path AS avatar_path FROM file_handlers fh WHERE is_cover = false) avatar ON u.avatar_id = avatar.av_id"

不走运！，这些都不适合我，我也不知道为什么。我错过了什么

---

[注意：请参阅]

我很确定您的问题是由于左连接，而不是合并。如果左连接不产生行，

cover.file_path

将以null结束，而不管是否合并

尝试将coalesce语句移动到更合适的位置：

1. COALESCE(fh.file_path, 'default image') AS file_path,
2. COALESCE(NULLIF(fh.file_path, ''), 'default image') AS file_path,
3. CASE WHEN fh.file_path IS NULL THEN 'default image' ELSE fh.file_path END AS file_path,

---

为我工作…耶

您的第一个合并示例是正确的，您是否尝试过直接在数据库上执行查询？它会返回什么？谢谢你的回复，我会直接在数据库中尝试一下，并让你知道..它会为postgres数据库中的文件路径返回空值，这也非常感谢。现在在postgres数据库中，它工作正常，但我仍然无法在查看页面中获得预期的结果。如果fh.file\u路径为空，则“默认图像”否则fh.file\u路径结束为file\u路径。。。对我有效，而不是联合（）。。。。顺便说一下，非常感谢奥利的帮助

sql = "SELECT DISTINCT(p.*), concat(u.first_name,' ',u.last_name) as fullname, concat('@',u.username) as username, coalesce(cover.file_path, 'default image path'), cover.file_type, avatar.avatar_path 
FROM products p
...

SELECT DISTINCT(p.*), concat(u.first_name,' ',u.last_name) as fullname, 
CASE 
  WHEN 
    cover.file_path IS NULL AND cover.file_type='image' 
  THEN 
    'DEFAULT IMAGE PATH' 
  WHEN
    cover.file_path IS NULL AND cover.file_type='audio' 
  THEN 
    'DEFAULT AUDIO PATH' 
  WHEN
    cover.file_path IS NULL AND cover.file_type='video' 
  THEN 
    'DEFAULT VIDEO PATH'
  ELSE 
    cover.file_path 
END,     
CASE 
  WHEN avatar.avatar_path IS NULL 
  THEN 'avatar.png' 
  ELSE avatar.avatar_path 
END
FROM products p
JOIN stores s ON s.id = p.store_id
JOIN users u ON u.id = s.user_id
LEFT OUTER JOIN (SELECT fh.product_id AS cp_id, fh.file_path AS file_path, fh.file_type AS  file_type FROM file_handlers fh WHERE is_cover = true) cover ON p.id = cover.cp_id
LEFT OUTER JOIN (SELECT fh.id AS av_id, fh.file_path AS avatar_path FROM file_handlers fh WHERE is_cover = false) avatar ON u.avatar_id = avatar.av_id