1.db.users.find()
select * from users
2.db.users.find({"age" : 27})
select * from users where age = 27
3.db.users.find({"username" : "joe", "age" : 27})
select * from users where username = "joe" and age = 27
4.db.users.find({}, {"username" : 1, "email" : 1})
select username, email from users
5.db.users.find({}, {"username" : 1, "_id" : 0})
// no case // 即时加上了列筛选，_id也会返回；必须显式的阻止_id返回
6.db.users.find({"age" : {"lte" : 30}})
select * from users where age >=18 and age <= 30 // lte(<=) gte(>=)
7.db.users.find({"username" : {"in" : [725, 542, 390]}})
select * from users where ticket_no in (725, 542, 390)
9.db.users.find({"ticket_no" : {"or" : [{"ticket_no" : 725}, {"winner" : true}]})
select * form users where ticket_no = 725 or winner = true
11.db.users.find({"id_num" : {"not": {"age" : 27}})
select * from users where not (age = 27)
13.db.users.find({"username" : {"exists" : true}})
select * from users where username is null
// 如果直接通过find({"username" : null})进行查询，那么连带"没有username"的纪录一并筛选出来
14.db.users.find({"name" : /joey?/i}) // 正则查询，value是符合PCRE的表达式
15.db.food.find({fruit : {size" : 3}})
// 对数组的查询, 查询数组元素个数是3的记录，slice" : 10}})
// 对数组的查询，只返回数组comments中的前十条，还可以{"slice" : [23, 10]}; 分别返回最后10条，和中间10条
19.db.people.find({"name.first" : "Joe", "name.last" : "Schmoe"}) // 嵌套查询
20.db.blog.find({"comments" : {"gte" : 5}}}})
// 嵌套查询，仅当嵌套的元素是数组时使用,
21.db.foo.find({"where当然是非常方便的，但效率低下。对于复杂查询，考虑的顺序应当是 正则 -> MapReduce -> where" : "function() { return this.x + this.y == 10; }"})
// $where可以支持javascript函数作为查询条件
23.db.foo.find().sort({"x" : 1}).limit(1).skip(10);
/ 返回第(10, 11]条，按"x"进行排序; 三个limit的顺序是任意的，应该尽量避免skip中使用large-number
24.db.user.find({"name":/ab/})
select * from user where name like "%ab%"
25.db.user.find({"name:/ABC\(v/})
select * from user where name like "%ABC(v%"
加双斜杠的原因是所查字段里面有括号，要转义一下
26.db.user.find({"name":/^ABC\(v/})
select * from user where name like "ABC(v%"