Mongodb从0到1系列二：数据库与集合操作文档、增删改查
5. 条件查询
上一节中提到了使用find()方法来查询文档，本节更加详细地介绍如何使用这个命令进行条件查询。find()方法语法如下：
db.collection.find(query, projection)
query ：可选，使用查询操作符指定查询条件
projection ：可选，使用投影操作符指定返回的键。查询时返回文档中所有键值， 只需省略该参数即可（默认省略）
先往student集合里插入一些文档：
MongoDB Enterprise > use test1
MongoDB Enterprise > db.student.insert({name:'Zhangsan',age:22,course:'Chinese'})
MongoDB Enterprise > db.student.insert({name:'Lisi',age:23,course:'computer'})
MongoDB Enterprise > db.student.insert({name:'Wangwu',age:24,course:'Chinese'})
MongoDB Enterprise > db.student.insert({name:'Zhaoliu',age:25,course:'Chinese'})
MongoDB Enterprise > db.student.insert({name:'Liuneng',age:26,course:'English'})
5.1 比较条件
示例1：查询年龄等于24的文档：
MongoDB Enterprise > db.student.find({age:24}).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf751420"),
"name" : "Wangwu",
"age" : 24,
"course" : "Chinese"
}
示例2: 查询年龄小于24的文档
MongoDB Enterprise > db.student.find({age:{$lt:24}}).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf75141e"),
"name" : "Zhangsan",
"age" : 22,
"course" : "Chinese"
}
{
"_id" : ObjectId("59703a654c9b439bdf75141f"),
"name" : "Lisi",
"age" : 23,
"course" : "computer"
}
5.2 and 条件
语法格式如下：>db.collection.find(
{
$and: [
{key1: value1}, {key2:value2}
]
}
).pretty()
示例3: 查询年龄小于24的，并且选课是computer的文档
MongoDB Enterprise > db.student.find( { $and:[{age:{$lt:24}}, {course:'computer'}] } ).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf75141f"),
"name" : "Lisi",
"age" : 23,
"course" : "computer"
}
其实，$and:[]是可以省略的，多个条件中间以逗号隔开即可，上面的查询可以简写如下：
MongoDB Enterprise > db.student.find( {age:{$lt:24}, course:'computer'}).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf75141f"),
"name" : "Lisi",
"age" : 23,
"course" : "computer"
}
示例4：查询满足三个条件的文档：
MongoDB Enterprise > db.student.find( {age:{$gt:23}, course:'Chinese', name:'Zhaoliu'}).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf751421"),
"name" : "Zhaoliu",
"age" : 25,
"course" : "Chinese"
}
5.3 or 条件
OR条件语法如下>db.collection.find(
{
$or: [
{key1: value1}, {key2:value2}
]
}
).pretty()
示例5：查询年龄小于24的，或者选课是English的文档
MongoDB Enterprise > db.student.find( { $or:[ { age:{$lt:24} }, {course:'English'} ] } ).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf75141e"),
"name" : "Zhangsan",
"age" : 22,
"course" : "Chinese"
}
{
"_id" : ObjectId("59703a654c9b439bdf75141f"),
"name" : "Lisi",
"age" : 23,
"course" : "computer"
}
{
"_id" : ObjectId("59703a674c9b439bdf751422"),
"name" : "Liuneng",
"age" : 26,
"course" : "English"
}
示例6：查询年龄小于24的，并且选课是computer的文档；或者选课是English的文档
MongoDB Enterprise > db.student.find( {
... $or:[
... { age:{$lt:24} , course:'computer' },
... { course:'English'}
... ]
... }
... ).pretty()
{
"_id" : ObjectId("59703a654c9b439bdf75141f"),
"name" : "Lisi",
"age" : 23,
"course" : "computer"
}
{
"_id" : ObjectId("59703a674c9b439bdf751422"),
"name" : "Liuneng",
"age" : 26,
"course" : "English"
}
5.4 查询固定列
在find()第二个参数中，指定键名且值为1或者true则是查询结果中显示的键；若值为0或者false，则为不显示键。文档中的键若在参数中没有指定，查询结果中将不会显示（_id例外）。这样我们就可以灵活显示声明来指定返回的键。示例7：只查询name键和age键：
MongoDB Enterprise > db.student.find({},{"name":1,"age":1,"_id":0})
{ "name" : "Zhangsan", "age" : 22 }
{ "name" : "Lisi", "age" : 23 }
{ "name" : "Wangwu", "age" : 24 }
{ "name" : "Zhaoliu", "age" : 25 }
{ "name" : "Liuneng", "age" : 26 }
若文档中不存在指定的key，则返回空：
MongoDB Enterprise > db.student.insert({name:'Liuneng222',age2:32,course:'English'})
MongoDB Enterprise > db.student.insert({name2:'Liuneng222',age2:32,course:'English'})
MongoDB Enterprise > db.student.find({},{"name":1,"age":1,"_id":0})
{ "name" : "Zhangsan", "age" : 22 }
{ "name" : "Lisi", "age" : 23 }
{ "name" : "Wangwu", "age" : 24 }
{ "name" : "Zhaoliu", "age" : 25 }
{ "name" : "Liuneng", "age" : 26 }
{ "name" : "Liuneng222" }
{ }
6. 大小写
mongodb大小写敏感，开启mongdb服务后，在数据库test2里创建三个collection，虽然名子“一样”，但由于大小写不一样，所以是不同的集合MongoDB Enterprise > use test2
switched to db test2
MongoDB Enterprise > db.createCollection("Student")
{ "ok" : 1 }
MongoDB Enterprise > db.createCollection("student")
{ "ok" : 1 }
MongoDB Enterprise > db.createCollection('STUDENT')
{ "ok" : 1 }
MongoDB Enterprise > show collections
STUDENT
Student
student
MongoDB Enterprise > db.createCollection("student")
{
"ok" : 0,
"errmsg" : "a collection 'test2.student' already exists",
"code" : 48,
"codeName" : "NamespaceExists"
}