如何在MongoDB聚合中使用$ ifNull？
$ifNull计算一个表达式，如果该表达式计算为非null值，则返回该表达式的值。
首先让我们创建一个包含文档的集合-
> db.demo372.insertOne({"FirstName":"Chris"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e591aea2ae06a1609a00af6")
}
> db.demo372.insertOne({"FirstName":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e591aef2ae06a1609a00af7")
}
> db.demo372.insertOne({"FirstName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e591af42ae06a1609a00af8")
}
> db.demo372.insertOne({"FirstName":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e591afb2ae06a1609a00af9")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo372.find();
这将产生以下输出-
{ "_id" : ObjectId("5e591aea2ae06a1609a00af6"), "FirstName" : "Chris" }
{ "_id" : ObjectId("5e591aef2ae06a1609a00af7"), "FirstName" : null }
{ "_id" : ObjectId("5e591af42ae06a1609a00af8"), "FirstName" : "David" }
{ "_id" : ObjectId("5e591afb2ae06a1609a00af9"), "FirstName" : null }以下是将$ifNull与聚合一起使用的查询-
> db.demo372.aggregate(
... [
... {
... $project: {
...
... FirstName: { $ifNull: [ "$FirstName", "NOT PROVIDED" ] }
... }
... }
... ]
... )这将产生以下输出-
{ "_id" : ObjectId("5e591aea2ae06a1609a00af6"), "FirstName" : "Chris" }
{ "_id" : ObjectId("5e591aef2ae06a1609a00af7"), "FirstName" : "NOT PROVIDED" }
{ "_id" : ObjectId("5e591af42ae06a1609a00af8"), "FirstName" : "David" }
{ "_id" : ObjectId("5e591afb2ae06a1609a00af9"), "FirstName" : "NOT PROVIDED" }