6.分页与排序

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分页与排序

1. limit() 返回指定条数的数据

a.查询出persons文档中前5条数据

> db.persons.find({},{_id:0}).limit(5){ "name" : "tom", "age" : 25, "email" : "214557457@qq.com", "c" : 75, "m" : 66, "e" : 97, "country" : "USA", "books" : [ "PHP", "JAVA", "EXTJS", "C++" ], "size" : 4 }{ "name" : "lili", "age" : 26, "email" : "344521457@qq.com", "c" : 75, "m" : 63, "e" : 97, "country" : "USA", "books" : [ "JS", "JAVA", "C#", "MONGODB" ], "size" : 4 }{ "name" : "zhangsan", "age" : 27, "email" : "2145567457@qq.com", "c" : 89, "m" : 86, "e" : 67, "country" : "China", "books" : [ "JS", "JAVA", "EXTJS", "MONGODB" ], "sex" : "M", "size" : 4 }{ "name" : "lisi", "age" : 26, "email" : "274521457@qq.com", "c" : 53, "m" : 96, "e" : 83, "country" : "China", "books" : [ "JS", "C#", "PHP", "MONGODB" ], "sex" : "M", "size" : 4 }{ "name" : "wangwu", "age" : 27, "email" : "65621457@qq.com", "c" : 45, "m" : 65, "e" : 99, "country" : "China", "books" : [ "JS", "JAVA", "C++", "MONGODB" ], "sex" : "M", "size" : 4 }

2.skip()返回指定数据的跨度

a.查询出persons文档中5~10条的数据

> db.persons.find({},{_id:0,name:1}).limit(5).skip(5){ "name" : "zhaoliu" }{ "name" : "piaoyingjun" }{ "name" : "lizhenxian" }{ "name" : "lixiaoli" }{ "name" : "zhangsuying" }

3.sort返回按照年龄排序的数据[1,-1]

1 : 顺序
-1：倒序

> db.persons.find({},{_id:0,name:1,age:1}).sort({age:1}){ "name" : "lixiaoli", "age" : 21 }{ "name" : "zhangsuying", "age" : 22 }{ "name" : "tom", "age" : 25 }{ "name" : "jim", "age" : 25 }{ "name" : "lili", "age" : 26 }{ "name" : "lisi", "age" : 26 }{ "name" : "piaoyingjun", "age" : 26 }{ "name" : "zhangsan", "age" : 27 }{ "name" : "wangwu", "age" : 27 }{ "name" : "zhaoliu", "age" : 27 }{ "name" : "lizhenxian", "age" : 27 }

注意：mongodb的key可以存储不同类型的数据，排序也就有了优先级:

最小值 > null > 数字 > 字符串 > 对象/文档 > 数组 > 二进制 > 对象ID > 布尔 > 日期 > 时间戳 > 正则 > 最大值

4.limit()和skip()完成分页

a.三条数据为一页进行分页

第一页：

> db.persons.find({},{_id:0,name:1}).limit(3).skip(0){ "name" : "tom" }{ "name" : "lili" }{ "name" : "zhangsan" }

第二页：

> db.persons.find({},{_id:0,name:1}).limit(3).skip(3){ "name" : "lisi" }{ "name" : "wangwu" }{ "name" : "zhaoliu" }

b.skip有性能问题，没有特殊情况下我们也可以换个思路

对文档进行重新结构设计:

增加Date键
在每次查询时候，前后台传值要把上次最后一个文档的日期保存下来
然后这样就可以做分页查询了：

db.person.find({date:{$gt:日期数值}}).limit(3)

个人建议：应该把软件重点放到便捷和精确查询上，而不是分页的性能上，因为用户最多不会翻页超过2页

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