哪些标签不在数据库中? [英] Which tags are not in the database?

问题描述

给出了一组用户指定的标签，如何确定带有1条SQL语句的标签表中哪些不是?

Given a collection of user specified tags how do I determine which ones are not in the tags table with 1 SQL Statement?

假设表架构为tags (id, tag)，并且我使用的是mysql，但我不知道有没有优化.

Assuming a table schema tags (id, tag) and I'm using mysql, if there's an optimization I'm unaware of.

谢谢

推荐答案

SELECT  Tag
FROM    UserSpecifiedTags
  LEFT OUTER JOIN AllTags ON UserSpecifiedTags.Tag = AllTags.Tag
WHERE   AllTags.Tag IS NULL

这应该返回您想要的.以我的经验，执行联接并查找不匹配的行比使用IN运算符要快 .

This should return what you want. In my experience, executing a join and looking for rows which don't have a match is much quicker than using the IN operator.

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