问题描述
在SQL中，可以添加1 = 1或1 = 0查询条件以本质上分别返回全部或不返回记录.我如何在mongo中做同样的事情?
In SQL, it's possible to add a 1=1 or 1=0 query condition to essentially return all or no records, respectively. How can I do the same in mongo?
推荐答案
正如评论中所回答的，mongo中1=1查询条件的等效项是使用空对象{}作为查询.例如，db.foos.find({}).
As answered in the comments, the equivalence of a 1=1 query condition in mongo is to use the empty object {} as the query. For example, db.foos.find({}).
通过将空数组传递给$in查询条件，可以在mongo中模拟1=0查询条件.例如，db.foos.find({ _id: { $in: [] } }).此外，使用在$where查询条件下始终返回false的javascript函数也可以.例如，db.foos.find({ $where: function() { return false } }).
A 1=0 query condition can be simulated in mongo by passing an empty array to the $in query condition. For example, db.foos.find({ _id: { $in: [] } }). Additionally, using a javascript function that always returns false in a $where query condition works as well. For example, db.foos.find({ $where: function() { return false } }).
与{ _id: 0 }或{ _id: null }相比，这两个条件对于1=0的附加好处是，即使0或null是集合中_id的有效值，它也将起作用.但是，我不确定这两个查询中的任何一个是否会带来额外的性能损失.
The added benefit of these two conditions for 1=0 compared to { _id: 0 } or { _id: null } is that it will work even if 0 or null are valid values for _id in your collection. However, I am not sure if either of these queries may carry additional performance penalties.
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