[Teach Youself SQL in 10 Minutes] The difference between HAVING and WHERE

iteye_1678

于 2011-04-13 16:15:57 发布

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本文详细解释了SQL中HAVING子句与WHERE子句的区别。WHERE子句在数据分组前进行过滤，而HAVING子句则在数据分组后进行过滤。文中强调了正确使用这两个子句的重要性，并给出了具体的使用建议。

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The difference between HAVING and WHERE Here's another way to look it: WHERE filters before data is grouped, and HAVING filters after data is grouped. This is an important distinction; rows that are eliminated by a WHERE clause will not be included in the group. This could change the calculated values which in turn could affect which groups are filtered based on the use of those values in the HAVING clause.

Using HAVING and WHERE HAVING is so similar to WHERE that most DBMSs treat them as the same thing if no GROUP BY is specified. Nevertheless, you should make that distinction yourself. Use HAVING only in conjunction with GROUP BY clauses. Use WHERE for standard row-level filtering.

BTW：

The only difference is that WHERE filters rows and HAVING filters groups.

The difference between HAVING and WHERE Here's another way to look it: WHERE filters before data is grouped, and HAVING filters after data is grouped.