MongoDB 基操教程(五) 檢索數據類型、limit、skip 和排序

檢索數據類型

找出數據中 num 字段類型爲 double 的數據，可以直接寫 double ，也可以寫在 MongoDB 中 double 類型指代的編碼，如下兩條語句是同義的：

db.test.find({'num': {'$type':1}})
db.test.find({'num': {'$type':'double'}})

關於數據的類型，以及對應的編碼圖，如下：（圖片來源於 菜鳥教程 MongoDB 模塊）

limit

通過 limit 限制返回的數據條數：

db.test.find().limit(2)
#返回數據：
{ "_id" : ObjectId("5d33d05a0536aa7970227ead"), "name" : "q1", "age" : 20, "sex" : "male", "num" : 34 }
{ "_id" : ObjectId("5d33d06b0536aa7970227eae"), "name" : "q2", "age" : 22, "sex" : "male", "num" : 62 }

skip

通過 skip 跳過前面幾條數據：

db.test.find().skip(2)
#返回數據
{ "_id" : ObjectId("5d33d06b0536aa7970227eaf"), "name" : "q3", "age" : 23, "sex" : "female", "num" : 92 }
{ "_id" : ObjectId("5d33d06d0536aa7970227eb0"), "name" : "q4", "age" : 24, "sex" : "female", "num" : 52 }

sort

指定字段升序、降序， 1 爲升序，-1 爲降序

db.test.find().sort({'num': 1})
{ "_id" : ObjectId("5d33d05a0536aa7970227ead"), "name" : "q1", "age" : 20, "sex" : "male", "num" : 34 }
{ "_id" : ObjectId("5d33d06d0536aa7970227eb0"), "name" : "q4", "age" : 24, "sex" : "female", "num" : 52 }
{ "_id" : ObjectId("5d33d06b0536aa7970227eae"), "name" : "q2", "age" : 22, "sex" : "male", "num" : 62 }
{ "_id" : ObjectId("5d33d06b0536aa7970227eaf"), "name" : "q3", "age" : 23, "sex" : "female", "num" : 92 }

db.test.find().sort({'num': -1})
{ "_id" : ObjectId("5d33d06b0536aa7970227eaf"), "name" : "q3", "age" : 23, "sex" : "female", "num" : 92 }
{ "_id" : ObjectId("5d33d06b0536aa7970227eae"), "name" : "q2", "age" : 22, "sex" : "male", "num" : 62 }
{ "_id" : ObjectId("5d33d06d0536aa7970227eb0"), "name" : "q4", "age" : 24, "sex" : "female", "num" : 52 }
{ "_id" : ObjectId("5d33d05a0536aa7970227ead"), "name" : "q1", "age" : 20, "sex" : "male", "num" : 34 }