MySQL：使用 UNION 与多个查询
我很想知道UNION是否比运行多个查询更好。
如果是这样，是否有时多个查询会更快或出于其他原因而成为首选？
I'm interested to know if a UNION is better than running multiple queries.
If so, are there times that multiple queries would be faster or preferred for other reasons?
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UNION ALL并且不排序输出，那么UNION 的性能应该与
假设查询是多个单独查询的性能
与您
UNION组合在一起的那些相同。UNION ALL并对数据进行排序，显然您会施加一些开销（尽管可能比您在其中排序您的应用程序要少）。
ALL关键字，MySQL 将执行以下额外工作DISTINCT-ing 你的结果。同样，这会带来额外的开销虽然可能比自己做的要少。
UNION ALLand don't sort the output, then theperformance of UNION should be more-or-less the same as the
performance of multiple separate queries assuming the queries are
identical to the ones you're
UNION-ing together.UNION ALLand sort the data, obviously you're imposing some overhead(although probably less than if you sorted in it your application).
ALLkeyword, MySQL will do the extra work ofDISTINCT-ing your results. Again, this imposes extra overheadalthough probably less than doing it yourself.
你问它是否“更好” - 我假设你指的是“性能方面”？
如果速度影响很小，您可能希望使用多个查询来保持代码更具可读性；而不是在一个联合中处理多个查询、做不同的事情。
当然，取决于用例。但从长远来看，可读且可理解的代码在更大的项目中可能是有价值的
You ask if it "is better" - I assume you are referring to "performance-wise"?
If the speed impact is minimal, you might want to prefer multiple queries to keep your code more readable; instead of dealing with multiple queries, doing different things, in one union.
Depends on the use case, for sure. But readable and understandable code might have it's worth in a bigger project in the long term