MongoDB的CURD基本操作(三)——查找

注：本文基于MongoDB 4.2.6编写

1、查找

• 查找集合所有文档

> db.bb.find()
{ "_id" : ObjectId("5ec29c1e6b02bc57526eb598"), "hello" : "ball", "animals" : { "pig" : 80, "cow" : 200, "dog" : 50 }, "may" : "May", "happy" : "ending" }
{ "_id" : ObjectId("5ec5384c6d59a15615f7df04"), "hello" : "ball", "name" : "jan", "happy" : "ending" }

为了便于阅读，我们可以对结果进行优化显示，

> db.bb.find().pretty()
{
	"_id" : ObjectId("5ec29c1e6b02bc57526eb598"),
	"hello" : "ball",
	"animals" : {
		"pig" : 80,
		"cow" : 200,
		"dog" : 50
	},
	"may" : "May",
	"happy" : "ending"
}
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan",
	"happy" : "ending"
}

• 查找特定文档

> db.bb.find({"name": "jan"}).pretty()
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan",
	"happy" : "ending"
}

• 查找并返回特定字段
  通过使用find的第二个参数来控制返回哪些字段，

> db.bb.find({"name": "jan"}, {"hello": 1, "name": 1}).pretty()
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan"
}

有一点需要注意的是，不能同时制定哪些返回和哪些不返回，否则会报错，

> db.bb.find({"name": "jan"}, {"hello": 0, "_id": 1}).pretty()
Error: error: {
	"ok" : 0,
	"errmsg" : "Projection cannot have a mix of inclusion and exclusion.",
	"code" : 2,
	"codeName" : "BadValue"
}

• 条件查询

> db.bb.find().pretty()
{
	"_id" : ObjectId("5ec684c3b97b4da914bd5e67"),
	"name" : "mike",
	"score" : 90
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e68"),
	"name" : "john",
	"score" : 90
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e69"),
	"name" : "lucy",
	"score" : 100
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96
}
> db.bb.find({score: {$gt: 90, $lt: 100}}).pretty()
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96
}
> db.bb.find({score: {$in:[96, 100]}}).pretty()
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e69"),
	"name" : "lucy",
	"score" : 100
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96
}
> db.bb.find({$or: [{name: "jack"}, {score: 96}]}).pretty()
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96
}

• 正则表达式

> db.bb.find({name: /ja.*/}).pretty()
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan",
	"happy" : "ending"
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96
}

• 数组查询
  对于匹配一个元素，直接键值对查询即可，如果需要同时匹配多个元素，要配合$all使用，

> db.bb.find({sport: "run"}).pretty()
{
	"_id" : ObjectId("5ec697c0b97b4da914bd5e6b"),
	"sport" : [
		"run",
		"swin",
		"jump",
		"tennis"
	]
}
> db.bb.find({sport: {$all: ["run", "swin"]}}).pretty()
{
	"_id" : ObjectId("5ec697c0b97b4da914bd5e6b"),
	"sport" : [
		"run",
		"swin",
		"jump",
		"tennis"
	]
}

但是如果数组中有多个元素，如下，记录了三人两次考试成绩，

> db.bb.find().pretty()
{
	"_id" : ObjectId("5ed6554230571733ccb3d67b"),
	"players" : [
		{
			"name" : "aa",
			"score" : 98
		},
		{
			"name" : "dan",
			"score" : 95
		},
		{
			"name" : "du",
			"score" : 100
		}
	]
}
{
	"_id" : ObjectId("5ed6579b30571733ccb3d67c"),
	"players" : [
		{
			"name" : "aa",
			"score" : 99
		},
		{
			"name" : "dan",
			"score" : 98
		},
		{
			"name" : "du",
			"score" : 90
		}
	]
}

如果需要找出du同学考试成绩大于95分的记录，

> db.bb.find({players: {name: "du", score: {$gt: 95}}}).pretty()
>

这种方式是无法查找出记录的，因为对于内嵌文档需要整个文档匹配，因此这是不可行的，

> db.bb.find({"players.name": "du", "players.score": {$gt: 95}}).pretty()
{
	"_id" : ObjectId("5ed6554230571733ccb3d67b"),
	"players" : [
		{
			"name" : "aa",
			"score" : 98
		},
		{
			"name" : "dan",
			"score" : 95
		},
		{
			"name" : "du",
			"score" : 100
		}
	]
}
{
	"_id" : ObjectId("5ed6579b30571733ccb3d67c"),
	"players" : [
		{
			"name" : "aa",
			"score" : 99
		},
		{
			"name" : "dan",
			"score" : 98
		},
		{
			"name" : "du",
			"score" : 90
		}
	]
}
> 

而直接使用内嵌文档字段查询，也是有问题的，因为这相当于在整个内嵌文档中进行匹配，由于两个文档中都有du同学，同时都有同学的分数大于95，因此返回了两个文档。

这时候就要需要$elemMatch，它可以匹配内嵌文档中单个文档，也就是能将查询条件分组，以组作为查询条件就不会出现刚才的情况，

> db.bb.find({players: {$elemMatch: {name: "du", score: {$gt: 95}}}}).pretty()
{
	"_id" : ObjectId("5ed6554230571733ccb3d67b"),
	"players" : [
		{
			"name" : "aa",
			"score" : 98
		},
		{
			"name" : "dan",
			"score" : 95
		},
		{
			"name" : "du",
			"score" : 100
		}
	]
}

• $slice切片
  主要用于返回符合条件的数组元素的子集，比如前面10个元素，最后10个，中间2-8个等条件，

> db.bb.find().pretty()
{
	"_id" : ObjectId("5ec697c0b97b4da914bd5e6b"),
	"sport" : [
		"run",
		"swin",
		"jump",
		"tennis"
	],
	"name" : "geek"
}
> db.bb.find({name: "geek"}, {sport: {$slice: -1}}).pretty()
{
	"_id" : ObjectId("5ec697c0b97b4da914bd5e6b"),
	"sport" : [
		"tennis"
	],
	"name" : "geek"
}
> db.bb.find({name: "geek"}, {sport: {$slice: [2,3]}}).pretty()
{
	"_id" : ObjectId("5ec697c0b97b4da914bd5e6b"),
	"sport" : [
		"jump",
		"tennis"
	],
	"name" : "geek"
}

• 查询结果进一步操作，limit、skip、sort
  limit用于限制返回结果数量，skip用于指定返回结果跳过的数量，sort用于对返回结果的排序

> db.bb.find({name: /^j.*/}).pretty()
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan",
	"happy" : "ending",
	"email" : "com"
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e68"),
	"name" : "john",
	"score" : 90
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96,
	"email" : "com"
}
> db.bb.find({name: /^j.*/}).limit(2).pretty()
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan",
	"happy" : "ending",
	"email" : "com"
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e68"),
	"name" : "john",
	"score" : 90
}
> db.bb.find({name: /^j.*/}).skip(2).pretty()
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96,
	"email" : "com"
}
> db.bb.find({name: /^j.*/}).sort({name: 1}).pretty()
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e6a"),
	"name" : "jack",
	"score" : 96,
	"email" : "com"
}
{
	"_id" : ObjectId("5ec5384c6d59a15615f7df04"),
	"hello" : "ball",
	"name" : "jan",
	"happy" : "ending",
	"email" : "com"
}
{
	"_id" : ObjectId("5ec684fab97b4da914bd5e68"),
	"name" : "john",
	"score" : 90
}

其中，sort参数中指定对某个字段进行升序(1)或者降序(-1)排序，

基本上find上能使用的查找语法在update更新操作上一样能使用。