3.查询中null的处理
null的匹配非常有意思：null不仅仅会匹配到指定键的值确实等于null的文档，并且还会匹配到查询所制定键不存在的文档。例如，插入下面这三个文档：
> db.users.insert({"name":"Tom","age":20,"job":"Sales"});
> db.users.insert({"name":"Sam","age":25,"job":"Manager"});
> db.users.insert({"name":"Jim","age":25});
> db.users.insert({"name":"Sam","age":25,"job":null});
> db.users.insert({"name":"Tom","age":20,"job":"Sales"});
> db.users.insert({"name":"Sam","age":25,"job":"Manager"});
> db.users.insert({"name":"Jim","age":25});
> db.users.insert({"name":"Sam","age":25,"job":null}); 现在我们用传统的方式来匹配job为null的文档：
> db.users.find({"job":null});
> db.users.find({"job":null}); 结果如下：
{ "_id" : ObjectId("4f0714e3edb28db4864be582"), "name" : "Jim", "age" : 25 }
{ "_id" : ObjectId("4f071542edb28db4864be583"), "name" : "Sam", "age" : 25, "job" : null }
{ "_id" : ObjectId("4f0714e3edb28db4864be582"), "name" : "Jim", "age" : 25 }
{ "_id" : ObjectId("4f071542edb28db4864be583"), "name" : "Sam", "age" : 25, "job" : null } 很明显，不仅将job确实为null的文档匹配出来，而且将没有job这个键的文档匹配出来了。很多时候这并不是我们的本意。有一个$exists操作符可以来解决这个问题。先看代码：
db.users.find({"job":{"$in":[null],"$exists":true}});
db.users.find({"job":{"$in":[null],"$exists":true}}); 结果如下：
{ "_id" : ObjectId("4f071542edb28db4864be583"), "name" : "Sam", "age" : 25, "job" : null }
{ "_id" : ObjectId("4f071542edb28db4864be583"), "name" : "Sam", "age" : 25, "job" : null }4.正则表达式
MongoDB使用Perl兼容的正则表达式库来匹配正则表达式。正则表达式本身非常强大，有专门写这个的书，我这里不针对这个，只是简单的举个例子。例如，忽略大小写情况下匹配name为Tom的文档：
db.users.find({"name":/tom/i});
db.users.find({"name":/tom/i});5.数组查询
数组查询大的思路：大部分情况下，数组的每个元素都可以是对应键的值。这么说肯定不好理解，举个例子，插入下面这样一个文档：
db.food.insert({"fruit":["apple","banana","peach"]});
db.food.insert({"fruit":["apple","banana","peach"]}); 现在执行下面三个查询，都会将这个文档匹配出来。
db.food.find({"fruit":"apple"});
db.food.find({"fruit":"banana"});
db.food.find({"fruit":"peach"});
db.food.find({"fruit":"apple"});
db.food.find({"fruit":"banana"});
db.food.find({"fruit":"peach"}); 如果需要匹配fruit键的值既有apple又有banana的文档，可以使用$all匹配符。现在我们假设有下面几个文档：
{ "_id" : ObjectId("4f071a10edb28db4864be584"), "fruit" : [ "apple", "banana", "peach" ] }
{ "_id" : ObjectId("4f071c5676285076f80ca7c7"), "fruit" : [ "apple", "banana" ] }
{ "_id" : ObjectId("4f071c6076285076f80ca7c8"), "fruit" : [ "apple" ] }
{ "_id" : ObjectId("4f071ce576285076f80ca7c9"), "fruit" : [ "banana", "apple" ] }
{ "_id" : ObjectId("4f071a10edb28db4864be584"), "fruit" : [ "apple", "banana", "peach" ] }
{ "_id" : ObjectId("4f071c5676285076f80ca7c7"), "fruit" : [ "apple", "banana" ] }
{ "_id" : ObjectId("4f071c6076285076f80ca7c8"), "fruit" : [ "apple" ] }
{ "_id" : ObjectId("4f071ce576285076f80ca7c9"), "fruit" : [ "banana", "apple" ] } 执行下面的查询：
db.food.find({"fruit":{"$all":["apple","banana"]}});
db.food.find({"fruit":{"$all":["apple","banana"]}}); 可以匹配到下面的三个文档：
{ "_id" : ObjectId("4f071a10edb28db4864be584"), "fruit" : [ "apple", "banana", "peach" ] }
{ "_id" : ObjectId("4f071c5676285076f80ca7c7"), "fruit" : [ "apple", "banana" ] }
{ "_id" : ObjectId("4f071ce576285076f80ca7c9"), "fruit" : [ "banana", "apple" ] }
{ "_id" : ObjectId |