mysql – 设置select中返回的空字段的默认值

给出两个表：

image(id, cat_id, name)
image_translation(id, title, desc, lang)

(名称和ID是唯一的)

查询以选择特定文件的所有翻译：

SELECT c.id           AS c__id, 
       c.cat_id       AS c__cat_id, 
       c.name         AS c__name, 
       c2.id          AS c2__id, 
       c2.title       AS c2__title, 
       c2.desc        AS c2__desc, 
       c2.lang        AS c2__lang, 
FROM   image c 
       LEFT JOIN image_translation c2 
              ON c.id = c2.id 
WHERE  ( c1.name = 'file.jpg' ) 

收益：

c__id | c__cat_id  | c2__id    | c__name   | c2__title     | cs__desc     | c2__lang
------+------------+-----------+-----------+---------------+--------------+--------- 
114   | 2          | 114       | file.jpg  | default title | default desc | en
114   | 2          | 114       | file.jpg  | NULL          | desc in de   | de
114   | 2          | 114       | file.jpg  | title in fr   | ''           | fr

要选择特定语言的特定文件的翻译,
显然我需要添加where子句.

但随后它返回默认行内容,其中可能包含翻译中的空字段.

如何将结果与默认值合并,并使用默认值覆盖empy字段？为lang en返回默认值：

SELECT c2.title       AS c2__title, 
       c2.desc        AS c2__desc, 
       c2.lang        AS c2__lang, 
FROM   image c 
       LEFT JOIN image_translation c2 
              ON c.id = c2.id 
WHERE  ( c1.name = 'file.jpg' ) 
AND WHERE ( c2.lang = 'en')

我期望的样本结果：

对于德语：

-- AND WHERE ( c2.lang = 'de' ) 

c__id | c__cat_id | c2__id    | c__name   | c2__title     | cs__desc     | c2__lang
-----+------------+-----------+-----------+---------------+--------------+--------- 
114  | 2          | 114       | file.jpg  | default title | desc in de   | de

对于法语：

-- AND WHERE ( c2.lang = 'fr' ) 

c__id | c__cat_id | c2__id    | c__name   | c2__title     | cs__desc     | c2__lang
-----+------------+-----------+-----------+---------------+--------------+--------- 
114  | 2          | 114       | file.jpg  | title in fr   | default desc | fr

如何在一个智能查询中正确执行？

(我使用MysqL和Doctrine ORM与I18N行为)

解决方法:

我不知道你是否要求这个简单的查询：

对于’fr’：

SELECT c2.title AS c2__title, 
       coalesce( nullif( c2.desc, '') , c3.desc )   AS desc, 
       coalesce( nullif( c2.lang, '') , c3.lang  )  AS lang, 
FROM   image c 
LEFT JOIN image_translation c2 
              ON c2.lang = 'fr' and c.id = c2.id 
LEFT JOIN image_translation c3
              ON c3.lang = 'en' and c.id = c3.id 
WHERE  ( c1.name = 'file.jpg' )