We will use the Null type here. Following are the null types with the alias −
| Type | Number | Alias |
|---|---|---|
| Double | 1 | “double” |
| String | 2 | “string” |
| Object | 3 | “object” |
| Array | 4 | “array” |
| Binary data | 5 | “binData” |
| Undefined | 6 | “undefined” |
| ObjectId | 7 | “objectId” |
| Boolean | 8 | “bool” |
| Date | 9 | “date” |
| Null | 10 | “null” |
| Regular Expression | 11 | “regex” |
Following is the syntax for type 10 i.e. null −
db.yourCollectionName.find({"yourFieldName":{ $type: 10 } });The above syntax will find only those documents which have null value. Let us first create a collection with documents −
> db.mongoDbEqualDemo.insertOne({"Age":34});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7e9121a844af18acdffa3")
}
> db.mongoDbEqualDemo.insertOne({"Age":""});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7e9161a844af18acdffa4")
}
> db.mongoDbEqualDemo.insertOne({"Age":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7e9191a844af18acdffa5")
}
> db.mongoDbEqualDemo.insertOne({"Age":56});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7e91e1a844af18acdffa6")
}
> db.mongoDbEqualDemo.insertOne({});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7e9261a844af18acdffa7")
}
> db.mongoDbEqualDemo.insertOne({"Age":null});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd7e92e1a844af18acdffa8")
}Following is the query to display all documents from a collection with the help of find() method −
> db.mongoDbEqualDemo.find().pretty();
This will produce the following output −
{ "_id" : ObjectId("5cd7e9121a844af18acdffa3"), "Age" : 34 }
{ "_id" : ObjectId("5cd7e9161a844af18acdffa4"), "Age" : "" }
{ "_id" : ObjectId("5cd7e9191a844af18acdffa5"), "Age" : null }
{ "_id" : ObjectId("5cd7e91e1a844af18acdffa6"), "Age" : 56 }
{ "_id" : ObjectId("5cd7e9261a844af18acdffa7") }
{ "_id" : ObjectId("5cd7e92e1a844af18acdffa8"), "Age" : null }Following is the query to check for equal to null values only −
> db.mongoDbEqualDemo.find({"Age":{ $type: 10 } });This will produce the following output −
{ "_id" : ObjectId("5cd7e9191a844af18acdffa5"), "Age" : null }
{ "_id" : ObjectId("5cd7e92e1a844af18acdffa8"), "Age" : null }