MySQL JOIN查询/ COUNT个问题

Question

I am trying to display a list of cookbooks which have been tagged with a specific tag. This is working, except I cannot also display the number of times the tag has been applied to each book by different users. I would like to sort by this number so a book tagged 'baking' for example, by multiple users, would rank higher than a book tagged 'baking' by only one user.

Here is a slightly simplified list of tables:

Table 1: cookbooks

ID, Title, Author, etc.

Table 2: recipes

ID, recipe_title, recipe, cookbook_id

Table 3: recipe_reviews

ID, recipe_id, star_rating, comments

Table 4: book_tags

tag_id, tag_text

Table 5: book_user_tag

user_id, book_id, tag_id

This is the test query as currently written:

SELECT cookbooks.*, COUNT(DISTINCT recipe_reviews.ID) as numreviews, COUNT( book_user_tag.tag_id) as numtags, AVG(recipe_reviews.star_rating) as average, book_tags.tag_text 
          FROM cookbooks 
          LEFT OUTER JOIN recipes ON cookbooks.ID = recipes.cookbook_id 
          LEFT OUTER JOIN recipe_reviews ON recipes.ID = recipe_reviews.recipe_id 
          LEFT OUTER JOIN book_user_tag ON cookbooks.ID = book_user_tag.book_id
          INNER JOIN book_tags ON book_user_tag.tag_id = book_tags.tag_id
          WHERE book_user_tag.tag_id = 69
          GROUP BY cookbooks.ID
          ORDER BY numtags DESC

UPDATE: This query ended up working when I debugged it and realized that a DISTINCT was required on the second count, and that it was trying to count the wrong variable; should have been user_id.

Answer 1

若要调试此问题，请尝试使用简单的SELECT *并删除GROUP BY子句和COUNT字段，这样可以看到在分组之前生成的原始数据，您可能会发现联接未按预期工作。