在MySQL中减少？

Question

我有主题(id*)和标签(id*,name)和链接表topic_tags(topicFk*,tagFk*).

现在我想选择每个主题,它包含所有好的标签(a,b,c),但没有坏标签(d,e,f).

我怎么做？

Answer 1

假设您的Topic_Tags表是唯一的,这可以回答您的确切问题 - 但可能无法推断您的实际问题:

SELECT
  TopicId
FROM Topic_Tags
JOIN Tags ON
  Topic_Tags.TagId = Tags.TagId
WHERE
  Tags.Name IN ('A', 'B', 'C', 'D', 'E', 'F')
GROUP BY
  TopicId
HAVING
  COUNT(*) = 3 
  AND MAX(Tags.Name) = 'C'

更通用的解决方案是:

SELECT 
    * 
FROM (
    SELECT
        TopicId
    FROM Topic_Tags
    JOIN Tags ON
        Topic_Tags.TagId = Tags.TagId
    WHERE
        Tags.Name IN ('A', 'B', 'C')
    GROUP BY
        TopicId
    HAVING
        COUNT(*) = 3 
) as GoodTags
LEFT JOIN (
    SELECT
        TopicId
    FROM Topic_Tags
    JOIN Tags ON
        Topic_Tags.TagId = Tags.TagId
    WHERE
        Tags.Name = 'D'
        OR Tags.Name = 'E'
        OR Tags.Name = 'F'
) as BadTags ON
    GoodTags.TopicId = BadTags.TopicId
WHERE
    BadTags.TopicId IS NULL