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