mongo的浮点型精度问题

luzq

在mongo计算分组求和，如10+25.66 = 35.6599999999 是这个结果。

这是浮点数机制决定的。所有系统的浮点数只要是按IEEE浮点数标准来的，都是这个样子。少数语言在显示的时候做了处理，比如PHP。其他平台就需要自己处理下。

不只是mongo，大部分的数据库都存在这个问题，各种合计都会有误差，为此有一个数据类型叫Decimal，使用文本存放数字，数据库合计之类的就没有误差了。当然程序中也要有这个数据类型，然后操作各种蛋痛各种不直观。

替代的方案是直接用int,以分为单位而不是以元为单位。当然前台显示时要各种处理，反正就没一个简洁的方案。

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zhangyaoxing

请使用MongoDB3.4，支持Decimal类型

22265

可使用 NumberDecimal

The decimal BSON typemongo shell decimal values are assigned and queried using the NumberDecimal() constructor. The following example adds a document containing gas prices to a gasprices collection:

db.gasprices.insert{ "_id" : 1, "date" : ISODate(), "price" : NumberDecimal("2.099"), "station" : "Quikstop", "grade" : "regular" }

The following query matches the document above:

db.gasprices.find( { price: NumberDecimal("2.099") } )

For more information on the decimal type, see NumberDecimal.

Converting Values to Decimal

A collection's values can be transformed to the decimal type by performing a one-time transformation or by modifying application logic to perform the transformation as it accesses records.

One-Time Collection Transformation

A collection can be transformed by iterating over all documents in the collection, converting the monetary value to the decimal type, and writing the document back to the collection.

NOTE

It is strongly advised to add the decimal value to the document as a new field and remove the old field later once the new field's values have been verified.

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[Model Monetary Data — MongoDB Manual 3.4](https://docs.mongodb.com/manual/tutorial/model-monetary-data/#numeric-decimal)