Question

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"

以上是我的postgresql数据库的原始sql查询。我的问题是，如果数据库中为null，我想显示默认的file_path。我尝试了三种方法：

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,

没有运气！它们都不适合我，我不知道为什么。我在这里缺少什么？

[注意：请参阅this screenshot]

Answer 1

我很确定你的问题是由于左连接而不是合并。如果左连接没有产生任何行，cover.file_path无论是否合并都会结束为空。

尝试将coalesce语句移动到更合适的位置：

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
...

Answer 2

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

对我有用......是啊！

COALESCE（FIELD，“DEFAULT VALUE”）不适用于postgresql + rails

2 个答案: