在MongoDB中查询空值？
要在MongoDB中查询空值，请使用$ne运算符。首先让我们创建一个包含文档的集合-
> db.queryingNullDemo.insertOne(
... {
... "StudentName":"Larry",
... "StudentDetails":
... {
... "StudentAge":21,
... "StudentSubject":"MongoDB"
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd00bec588d4a6447b2e05f")
}
> db.queryingNullDemo.insertOne(
... {
... "StudentName":"Sam",
... "StudentDetails":
... {
... "StudentAge":23,
... "StudentSubject":null
... }
... }
... );
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd00c00588d4a6447b2e060")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.queryingNullDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd00bec588d4a6447b2e05f"),
"StudentName" : "Larry",
"StudentDetails" : {
"StudentAge" : 21,
"StudentSubject" : "MongoDB"
}
}
{
"_id" : ObjectId("5cd00c00588d4a6447b2e060"),
"StudentName" : "Sam",
"StudentDetails" : {
"StudentAge" : 23,
"StudentSubject" : null
}
}以下是如何查询null-
> db.queryingNullDemo.find({'StudentDetails.StudentSubject': {$ne: null}});这将产生以下输出-
{ "_id" : ObjectId("5cd00bec588d4a6447b2e05f"),
"StudentName" : "Larry",
"StudentDetails" : { "StudentAge" : 21, "StudentSubject" : "MongoDB" } }