November 2021
Intermediate to advanced
410 pages
12h 1m
Chinese
Content preview from MongoDB权威指南（第3版）
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查询
｜
47
键的值减
1
。这样，只需用一条简单的查询语句就能检查出哪些商品处于缺货状态了。
> db.stock.find({"in_stock" : 0})
4.2
查询条件
查询不仅能像上一节描述的那样进行精确匹配，还可以匹配更加复杂的条件，比如范围、
OR
子句以及取反。
4.2.1
查询条件
"$lt"
、
"$lte"
、
"$gt"
和
"$gte"
都属于比较运算符，分别对应
<
、
<=
、
>
和
>=
。可以将
它们组合使用以查找一个范围内的值。例如，要查询
18
到
30
岁的用户
，可以进行如下
操作：
> db.users.find({"age" : {"$gte" : 18, "$lte" : 30}})
这样就可以查找到
"age"
字段大于等于
18
并且小于等于
30
的所有文档了。
这类范围查询通常对日期非常有用。例如，要查找在
2007
年
1
月
1
日前注册的用户
，可
以这样做：
> start = new Date("01/01/2007")
> db.users.find({"registered" : {"$lt" : start}})
根据创建和存储日期的方式，精确匹配可能用处不大，因为日期是以毫秒精度存储的。通
常，你需要的是一整天、一周或一个月的数据，因此需要使用范围查询。
对于文档键值不等于某个特定值的情况，就要使用另外一种条件运算符
"$ne"
了，它表示
“不相等”。如果想找到所有用户名不为
joe
的用户，可以像下面这样查询：
> db.users.find({"username" : {"$ne" : "joe"}})
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.