WHERE 和 HAVING 之间的语义差异是什么？
让我们先把GROUP BY放在一边。在普通查询（没有GROUP BY）中，语义差异是什么？为什么这个答案有效？ （将别名放在 HAVING 子句中，而不是 WHERE 中）
Let's put GROUP BY aside for a second. In normal queries (without GROUP BY), what is the semantic difference? Why does this answer work? (put an alias in a HAVING clause instead of WHERE)
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（3）
HAVING对汇总行进行操作 -WHERE在应用GROUP BY之前对整个表进行操作。 （您不能将GROUP BY放在一边，HAVING是保留与GROUP BY一起使用的子句 - 省略GROUP BY不会改变幕后发生的隐式操作）。还需要注意的是，正因为如此，
WHERE可以使用索引，而HAVING则不能。 （在超级简单的未分组结果集中，理论上您可以使用HAVING索引，但我从未见过实际以这种方式实现的查询优化器）。HAVINGoperates on the summarized row -WHEREis operating on the entire table before theGROUP BYis applied. (You can't putGROUP BYaside,HAVINGis a clause reserved for use withGROUP BY- leaving out theGROUP BYdoesn't change the implicit action that is occurring behind the scenes).It's also important to note that because of this,
WHEREcan use an index whileHAVINGcannot. (In super trivial un-grouped result sets you could theoretically use an index forHAVING, but I've never seen a query optimizer actually implemented in this way).MySQL 评估查询直至并包括
WHERE子句，然后使用HAVING子句对其进行过滤。这就是为什么HAVING可以识别列别名，而WHERE却不能。通过省略 GROUP BY 子句，我相信您只是告诉查询不要对任何结果进行分组。
MySQL evaluates the query up to and including the
WHEREclause, then filters it with theHAVINGclause. That's whyHAVINGcan recognize column aliases, whereasWHEREcan't.By omitting the
GROUP BYclause, I believe you simply tell the query not to group any of your results.概括地说，
WHERE过滤进入查询的数据（数据库表），而HAVING过滤查询的输出。WHERE子句中的语句只能引用表（和其他外部数据源），而HAVING子句中的语句只能引用查询生成的数据。Very broadly,
WHEREfilters the data going into the query (the DB tables), whileHAVINGfilters the output of the query.Statements in the
WHEREclause can only refer to the tables (and other external data sources), while statements in theHAVINGclause can only refer to the data produced by the query.