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法律研究如何在 MongoDB 中查询字段为 null 或未设置的记录？
下面两种情况的使用指南 −
情况 1 − 当字段存在且设置为 null 时的语法如下。
db.yourCollectionName.count({yourFieldName: null});情况 1 − 当字段不存在且未设置时的语法如下。
db.yourCollectionName.count({yourFieldName: {$exists: false}});为理解上述语法，让我们使用文档创建一个集合。通过文档创建集合的查询如下 −
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Larry","EmployeeAge":null,"EmployeeSalary":18500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a995c6cea1f28b7aa07fe")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Bob","EmployeeAge":21,"EmployeeSalary":23500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a99836cea1f28b7aa07ff")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Carol","EmployeeSalary":45500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a999b6cea1f28b7aa0800")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Mike","EmployeeAge":null,"EmployeeSalary":45500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a99bb6cea1f28b7aa0801")
}
> db.fieldIsNullOrNotSetDemo.insertOne({"EmployeeName":"Ramit","EmployeeSalary":85500});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c8a99d76cea1f28b7aa0802")
}借助 find() 方法显示来自集合的所有文档。查询如下 −
> db.fieldIsNullOrNotSetDemo.find().pretty();
输出如下 −
{
"_id" : ObjectId("5c8a995c6cea1f28b7aa07fe"),
"EmployeeName" : "Larry",
"EmployeeAge" : null,
"EmployeeSalary" : 18500
}
{
"_id" : ObjectId("5c8a99836cea1f28b7aa07ff"),
"EmployeeName" : "Bob",
"EmployeeAge" : 21,
"EmployeeSalary" : 23500
}
{
"_id" : ObjectId("5c8a999b6cea1f28b7aa0800"),
"EmployeeName" : "Carol",
"EmployeeSalary" : 45500
}
{
"_id" : ObjectId("5c8a99bb6cea1f28b7aa0801"),
"EmployeeName" : "Mike",
"EmployeeAge" : null,
"EmployeeSalary" : 45500
}
{
"_id" : ObjectId("5c8a99d76cea1f28b7aa0802"),
"EmployeeName" : "Ramit",
"EmployeeSalary" : 85500
}情况 1 −
字段 EmployeeAge 存在且设置为 null。
查询如下 −
> db.fieldIsNullOrNotSetDemo.count({EmployeeAge: null});输出如下 −
4
情况 2 −
字段 'EmployeeAge' 不存在且未设置。查询如下 −
> db.fieldIsNullOrNotSetDemo.count({EmployeeAge: {$exists: false}});输出如下 −
2
更新日期： 30-Jul-2019
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