show dbs
show tables
show collections
use user
db.createCollection('config_collection')
use user
db.user_collection.insert({username:'tom'})
db.user_collection.insertOne({username: jhoe})
use user
db.dropDatabase()
use user
db.user_collection.drop()
use user
db.config_collection.find()
db.config_collection.findOne()
db.config_collection.find().pretty() //格式化显示
