Question

我有两张相关的表格;类别和订阅。类别＆gt;订阅的关系并不总是存在。所以我需要这样的东西：

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

有没有办法写这样的条件？

Answer 1

您可以在逻辑上重写您的条件：

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'

Answer 2

由于if / else子句中的所有术语都是逻辑与，因此您可以简单地对它们进行逻辑分组。

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...)

WHERE条件下的IF语句

2 个答案: