如何查询MongoDB中字段为空或未设置的记录？

让我们解决两种情况 -

情况 1 - 当该字段存在并设置为 null 时，语法如下。

db.yourCollectionName.count({yourFieldName: null});

情况 1 - 当该字段不存在且未设置时，语法如下。

db.yourCollectionName.count({yourFieldName: {$exists: false}});

为了理解上述两种语法，让我们用文档创建一个集合。使用文档创建集合的查询如下 -

> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Larry","EmployeeAge":null,"EmployeeSalary":18500});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c8a995c6cea1f28b7aa07fe")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Bob","EmployeeAge":21,"EmployeeSalary":23500});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c8a99836cea1f28b7aa07ff")
}

> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Carol","EmployeeSalary":45500});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c8a999b6cea1f28b7aa0800")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Mike","EmployeeAge":null,"EmployeeSalary":45500});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c8a99bb6cea1f28b7aa0801")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Ramit","EmployeeSalary":85500});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5c8a99d76cea1f28b7aa0802")
}

在find()方法的帮助下显示集合中的所有文档。查询如下 -

> db.fieldIsNullOrNotSetDemo.find().pretty();

以下是输出 -

{
   "_id" : ObjectId("5c8a995c6cea1f28b7aa07fe"),
   "EmployeeName" : "Larry",
   "EmployeeAge" : null,
   "EmployeeSalary" : 18500
}
{
   "_id" : ObjectId("5c8a99836cea1f28b7aa07ff"),
   "EmployeeName" : "Bob",
   "EmployeeAge" : 21,
   "EmployeeSalary" : 23500
}
{
   "_id" : ObjectId("5c8a999b6cea1f28b7aa0800"),
   "EmployeeName" : "Carol",
   "EmployeeSalary" : 45500
}
{
   "_id" : ObjectId("5c8a99bb6cea1f28b7aa0801"),
   "EmployeeName" : "Mike",
   "EmployeeAge" : null,
   "EmployeeSalary" : 45500
}
{
   "_id" : ObjectId("5c8a99d76cea1f28b7aa0802"),
   "EmployeeName" : "Ramit",
   "EmployeeSalary" : 85500
}

案例 1 -

字段 EmployeeAge 存在并设置为 null。

查询如下 -

> db.fieldIsNullOrNotSetDemo.count({EmployeeAge: null});

以下是输出 -

4

案例 2 -

字段“EmployeeAge”不存在且未设置。查询如下 -

> db.fieldIsNullOrNotSetDemo.count({EmployeeAge: {$exists: false}});

以下是输出 -

2