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我自己也在学习 MongoDB，我发现自己处于一个奇怪的境地。这是查询#1

  1. 当我db.players.find({$or: [{"position":"Left Wing"}, {"position":"Right Wing"}]}).pretty()在示例数据集上运行时，结果如下：

    "_id" : ObjectId("57d9a212698d3e0bd0f5a245"),
"position" : "Right Wing",
"id" : 8465166,
"weight" : 200,
"height" : "6' 0\"",
"imageUrl" : "http://1.cdn.nhle.com/photos/mugs/8465166.jpg",
"birthplace" : "Seria, BRN",
"age" : 37,
"name" : "Craig Adams",
"birthdate" : "April 26, 1977",
"number" : 27


"_id" : ObjectId("57d9a212698d3e0bd0f5a246"),
"position" : "Right Wing",
"id" : 8475761,
"weight" : 195,
"height" : "6' 2\"",
"imageUrl" : "http://1.cdn.nhle.com/photos/mugs/8475761.jpg",
"birthplace" : "Gardena, CA, USA",
"age" : 23,
"name" : "Beau Bennett",
"birthdate" : "November 27, 1991",
"number" : 19


"_id" : ObjectId("57d9a212698d3e0bd0f5a247"),
"position" : "Left Wing",
"id" : 8471260,
"weight" : 202,
"height" : "6' 1\"",
"imageUrl" : "http://3.cdn.nhle.com/photos/mugs/8471260.jpg",
"birthplace" : "Meadow Lake, SK, CAN",
"age" : 29,
"name" : "Blake Comeau",
"birthdate" : "February 18, 1986",
"number" : 17


"_id" : ObjectId("57d9a212698d3e0bd0f5a249"),
"position" : "Right Wing",
"id" : 8471703,
"twitterURL" : "https://twitter.com/S9Downie",
"weight" : 191,
"height" : "5' 11\"",
"imageUrl" : "http://1.cdn.nhle.com/photos/mugs/8471703.jpg",
"birthplace" : "Newmarket, ON, CAN",
"twitterHandle" : "S9Downie",
"age" : 28,
"name" : "Steve Downie",
"birthdate" : "April 03, 1987",
"number" : 23


"_id" : ObjectId("57d9a212698d3e0bd0f5a24a"),
"position" : "Right Wing",
"id" : 8466393,
"weight" : 205,
"height" : "6' 1\"",
"imageUrl" : "http://2.cdn.nhle.com/photos/mugs/8466393.jpg",
"birthplace" : "Laval, QC, CAN",
"age" : 35,
"name" : "Pascal Dupuis",
"birthdate" : "April 07, 1979",
"number" : 9


"_id" : ObjectId("57d9a212698d3e0bd0f5a24b"),
"position" : "Right Wing",
"id" : 8471887,
"weight" : 189,
"height" : "5' 11\"",
"imageUrl" : "http://1.cdn.nhle.com/photos/mugs/8471887.jpg",
"birthplace" : "Sollentuna, SWE",
"age" : 28,
"name" : "Patric Hornqvist",
"birthdate" : "January 01, 1987",
"number" : 72


"_id" : ObjectId("57d9a212698d3e0bd0f5a24c"),
"position" : "Left Wing",
"id" : 8470543,
"weight" : 195,
"height" : "6' 0\"",
"imageUrl" : "http://3.cdn.nhle.com/photos/mugs/8470543.jpg",
"birthplace" : "Regina, SK, CAN",
"age" : 35,
"name" : "Chris Kunitz",
"birthdate" : "September 26, 1979",
"number" : 14


"_id" : ObjectId("57d9a212698d3e0bd0f5a24f"),
"position" : "Left Wing",
"id" : 8474102,
"twitterURL" : "https://twitter.com/DP_57",
"weight" : 200,
"height" : "6' 0\"",
"imageUrl" : "http://3.cdn.nhle.com/nhl/en/v3/photos/mugs/8474102.jpg",
"birthplace" : "Sherbrooke, QC, CAN",
"twitterHandle" : "DP_57",
"age" : 26,
"name" : "David Perron",
"birthdate" : "May 28, 1988",
"number" : 39

    抱歉，格式化 JSON 文件很困难。请假设上面的较大间隙为括号所在的位置。

  2. 当我运行db.players.find({"position":"Left Wing"} || {"position":"Right Wing"}).pretty(),结果是：

    "_id" : ObjectId("57d9a212698d3e0bd0f5a247"),
"position" : "Left Wing",
"id" : 8471260,
"weight" : 202,
"height" : "6' 1\"",
"imageUrl" : "http://3.cdn.nhle.com/photos/mugs/8471260.jpg",
"birthplace" : "Meadow Lake, SK, CAN",
"age" : 29,
"name" : "Blake Comeau",
"birthdate" : "February 18, 1986",
"number" : 17


"_id" : ObjectId("57d9a212698d3e0bd0f5a24c"),
"position" : "Left Wing",
"id" : 8470543,
"weight" : 195,
"height" : "6' 0\"",
"imageUrl" : "http://3.cdn.nhle.com/photos/mugs/8470543.jpg",
"birthplace" : "Regina, SK, CAN",
"age" : 35,
"name" : "Chris Kunitz",
"birthdate" : "September 26, 1979",
"number" : 14


"_id" : ObjectId("57d9a212698d3e0bd0f5a24f"),
"position" : "Left Wing",
"id" : 8474102,
"twitterURL" : "https://twitter.com/DP_57",
"weight" : 200,
"height" : "6' 0\"",
"imageUrl" : "http://3.cdn.nhle.com/nhl/en/v3/photos/mugs/8474102.jpg",
"birthplace" : "Sherbrooke, QC, CAN",
"twitterHandle" : "DP_57",
"age" : 26,
"name" : "David Perron",
"birthdate" : "May 28, 1988",
"number" : 39

考虑到它们都是OR操作，为什么会出现这种差异？请帮助我理解它。

IntelliJ 上的 Windows 10/MongoDB

4

2 回答 2

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根据我对 mongo 知识的理解。

  • || 操作员命令以下列方式工作

  • 一个 || b -> 如果 a 为真，它只是将值 a 传递给 db.players.find(a) 并忽略 b。

  • 在您的情况下，这里的 'a' 是指获取 {"position":"Left Wing"} 因此它仅检索“Left Wing”文档。

  • 其中 $or: 为每个条件准备索引并获取“左翼”和“右翼”的组合结果。

在 mongo shell 上尝试以下命令，我认为它提供了一些更好的主意。

> {$or: [(1==1), (2==1)]}
 [ true, false ]
> (1==1) || (2==1)
 true
> {$or: [(2==1), (1==1)]}
 [ false, true ]
> (2==1) || (1==1)
 true

希望能帮助到你。

谢谢，
拉纳

于 2016-09-14T20:24:55.727 回答
1

参数 of{"position":"Left Wing"} || {"position":"Right Wing"}是一个 JavaScript 表达式，其计算结果为，{"position":"Left Wing"}因为该术语是真实的。这就是您实际传递给 的内容find，这就是为什么您只获取该"Left Wing"查询中的文档的原因。

您可以通过执行以下操作在 shell 中看到这一点：

> var query = {"position":"Left Wing"} || {"position":"Right Wing"}
> query
{ "position" : "Left Wing" }

但是您也可以简化$or查询以使用$in：

db.players.find({"position": {$in: ["Left Wing", "Right Wing"]}})
于 2016-09-15T01:35:46.023 回答