Mysql WHERE条件下的IF语句

我有两个相关的表格；类别和订阅。类别->订阅的关系并不总是存在。所以我需要像这样的东西：

SELECT * FROM categories
LEFT JOIN subscriptions 
ON subscriptions.category_id = categories.category_id 
WHERE
 IF EXISTS(subscriptions.is_enable)
  categories.type = 'premium' AND
  subscriptions.is_enable = 1
ELSE
  categories.type = 'premium' AND

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有没有办法这样写条件？

因为if/else子句中的所有术语都是逻辑AND，所以您可以简单地将它们逻辑地分组

SELECT * FROM categories
LEFT JOIN subscriptions 
ON subscriptions.category_id = categories.category_id 
WHERE
  (EXISTS(subscriptions.is_enable) AND 
  categories.type = 'premium' AND
  subscriptions.is_enable = 1)
OR
  (categories.type = 'premium' AND...)

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您可以按如下逻辑重写您的条件：

SELECT * FROM categories
LEFT JOIN subscriptions 
ON subscriptions.category_id = categories.category_id 
WHERE (subscriptions.is_enable IS NULL OR subscriptions.is_enable = 1)
  AND categories.type = 'premium'

---

谢谢这就是我需要的。