MongoDB：如何查询字段为空或未设置的记录？
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MongoDB: How to query for records where field is null or not set?
提问by Andrew
I have an Emaildocument which has a sent_atdate field:
我有一个Email包含sent_at日期字段的文档：
{
'sent_at': Date( 1336776254000 )
}
If this Emailhas not been sent, the sent_atfield is either null, or non-existant.
如果Email尚未发送，则该sent_at字段为空或不存在。
I need to get the count of all sent/unsent Emails. I'm stuck at trying to figure out the right way to query for this information. I think this is the right way to get the sent count:
我需要获取所有已发送/未发送的计数Emails。我一直在试图找出查询此信息的正确方法。我认为这是获取发送计数的正确方法：
db.emails.count({sent_at: {$ne: null}})
But how should I get the count of the ones that aren't sent?
但是我应该如何获得未发送的数量？
回答by jdi
If the sent_at field is not there when its not set then:
如果 sent_at 字段在未设置时不存在，则：
db.emails.count({sent_at: {$exists: false}})
If its there and null, or not there at all:
如果它在那里并且为空，或者根本不存在：
db.emails.count({sent_at: null})
回答by tvs
If you want to ONLY count the documents with sent_atdefined with a value of null(don't count the documents with sent_atnot set):
如果您只想计算sent_at定义为值null的文档（不要计算sent_at未设置的文档）：
db.emails.count({sent_at: { $type: 10 }})
回答by Anil Tallam
Use:
用：
db.emails.count({sent_at: null})
Which counts all emails whose sent_at property is null or is not set. The above query is same as below.
计算其 sent_at 属性为空或未设置的所有电子邮件。上面的查询与下面的相同。
db.emails.count($or: [
{sent_at: {$exists: false}},
{sent_at: null}
])
回答by Jim Horng
Seems you can just do single line:
似乎你只能做单行：
{ "sent_at": null }
回答by Vivek Maraina
You can also try this:
你也可以试试这个：
db.emails.find($and:[{sent_at:{$exists:true},'sent_at':null}]).count()
