问题描述
我看到了很多查询，内容如下.
I have seen many queries with something as follows.
Select 1
From table
此1是什么意思，它将如何执行以及返回什么?
What does this 1 mean, how will it be executed and, what will it return?
还可以在哪种类型的方案中使用它?
Also, in what type of scenarios, can this be used?
推荐答案
SELECT 1 FROM TABLE_NAME的意思是从表中返回1".它本身并不十分明显，因此通常将它与WHERE并经常与EXISTS一起使用(如@gbn所述，这不一定是最佳实践，但是，即使有，它也足够普遍地被注意到.并没有真正的意义(也就是说，我会使用它，因为其他人会使用它，并且它会立即变得更加明显".当然，这可能是粘性鸡与蛋的问题，但我通常不会对此进行介绍)).
SELECT 1 FROM TABLE_NAME means, "Return 1 from the table". It is pretty unremarkable on its own, so normally it will be used with WHERE and often EXISTS (as @gbn notes, this is not necessarily best practice, it is, however, common enough to be noted, even if it isn't really meaningful (that said, I will use it because others use it and it is "more obvious" immediately. Of course, that might be a viscous chicken vs. egg issue, but I don't generally dwell)).
SELECT * FROM TABLE1 T1 WHERE EXISTS (
SELECT 1 FROM TABLE2 T2 WHERE T1.ID= T2.ID
);
基本上，以上代码将返回表1的所有内容，而表1具有表2的对应ID.(显然，这是一个人为的示例，但我相信它传达了这一想法.就我个人而言，我可能会将其作为，因为我认为 FAR 对读者更为明确，除非有明显的迫不得已的理由不这样做.
Basically, the above will return everything from table 1 which has a corresponding ID from table 2. (This is a contrived example, obviously, but I believe it conveys the idea. Personally, I would probably do the above as SELECT * FROM TABLE1 T1 WHERE ID IN (SELECT ID FROM TABLE2); as I view that as FAR more explicit to the reader unless there were a circumstantially compelling reason not to).
实际上直到现在我都忘记了一种情况.在尝试从外部语言确定数据库中是否存在值的情况下，有时会使用SELECT 1 FROM TABLE_NAME.与选择单个列相比，这不会带来明显的好处，但是，根据实现的不同，与执行SELECT *相比，它可能会带来实质性的收益，这仅仅是因为通常情况下，数据库返回一种语言的列越多，数据库返回的语言越多.较大的数据结构，这又意味着将花费更多时间.
There actually is one case which I forgot about until just now. In the case where you are trying to determine existence of a value in the database from an outside language, sometimes SELECT 1 FROM TABLE_NAME will be used. This does not offer significant benefit over selecting an individual column, but, depending on implementation, it may offer substantial gains over doing a SELECT *, simply because it is often the case that the more columns that the DB returns to a language, the larger the data structure, which in turn mean that more time will be taken.
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