mysql - 如何修复此 LEFT JOIN 查询？

Question

我有三个表:

• 产品(product_id，标题)
• 评论(comment_id、product_id、user_id、comment、post_date)
• 书签(user_id、product_id、read_date)

对于 products 表中的每个 product_id，我希望检索具有相同 product_id 的评论数，其 post_date 值大于书签表中共享此 product_id 且 user_id= 的行的 read_date 值22.

如果书签表中不存在这样的一行，我想检索该 product_id 的评论总数，而不考虑 read_date。

目前为止

SELECT p.product_id, COUNT( c.comment_id ) comment_count
FROM products p
LEFT JOIN bookmarks b, comments c ON b.product_id = c.product_id
AND b.user_id =22
AND (
c.post_date > b.read_date
)
AND p.product_id = c.product_id
GROUP BY c.product_id
ORDER BY comment_count DESC

这没有给我预期的结果。我怎样才能修改它以使其执行我想要的操作？

Answer 1

它对你有用吗？

SELECT p.product_id, 
COUNT(CASE 
  WHEN b.read_date IS NOT NULL AND c.post_date >b.read_date THEN c.comment_id
  WHEN b.read_date IS NULL THEN c.comment_id
  ELSE NULL //optional, CASE has default ELSE NULL
END) as comment_count
FROM products p
LEFT JOIN bookmarks b ON (b.product_id = p.product_id AND b.user_id=22) 
LEFT JOIN comments c ON (p.product_id = c.product_id)
GROUP BY p.product_id
ORDER BY comment_count DESC

更新 GROUP BY c.product_id 更改为 GROUP BY p.product_id