How to realize Field judgment in MongoDB
Today, I will talk to you about how to achieve field judgment in MongoDB, many people may not know much about it. In order to make you understand better, the editor has summarized the following content for you. I hope you can get something according to this article.
Use "$exists" to determine whether a field exists, if set to true means it exists, and if it does not exist along with false.
Example: query information about parents personnel
> db.emp.find ({"parents": {"$exists": true}}) .pretty ()
{
"_ id": ObjectId ("599148bd0184ff511bf02b91")
"name": "Lin A"
"sex": "male"
"age": 22
"sal": 8000
"loc": "Beijing"
"course": [
"Chinese"
"Mathematics"
"English"
"Music"
"politics"
]
"parents": [
{
"name": "Lin A Father"
"age": 50
Job: the Peasant
}
{
"name": "Lin A Mother"
"age": 49
"job": "worker
}
]
}
Example: query information that does not have course personnel
> db.emp.find ({"course": {"$exists": false}}) .pretty ()
{
"_ id": ObjectId ("599108423268c8e84253be26")
"name": "Zhao Yi"
"sex": "male"
"age": 30
"sal": 1000
"loc": "Beijing"
}
You can use such queries to filter unwanted data. It is strongly recommended that the composition of the data be consistent.
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