检查MongoDB文档中字段的值是否存在？
要检查MongoDB文档中字段的值是否存在，可以find()与$exists运算符一起使用。首先让我们创建一个包含文档的集合-
> db.checkIfValueDemo.insertOne({"PlayerName":"John Smith","PlayerScores":[5000,98595858,554343]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6f507af8e7a4ca6b2ad98")
}
> db.checkIfValueDemo.insertOne({"PlayerName":"John Doe","PlayerScores":[]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6f512af8e7a4ca6b2ad99")
}
> db.checkIfValueDemo.insertOne({"PlayerName":"Carol Taylor","PlayerScores":[7848474,8746345353]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6f521af8e7a4ca6b2ad9a")
}
> db.checkIfValueDemo.insertOne({"PlayerName":"David Miller","PlayerScores":[]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cc6f531af8e7a4ca6b2ad9b")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.checkIfValueDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cc6f507af8e7a4ca6b2ad98"),
"PlayerName" : "John Smith",
"PlayerScores" : [
5000,
98595858,
554343
]
}
{
"_id" : ObjectId("5cc6f512af8e7a4ca6b2ad99"),
"PlayerName" : "John Doe",
"PlayerScores" : [ ]
}
{
"_id" : ObjectId("5cc6f521af8e7a4ca6b2ad9a"),
"PlayerName" : "Carol Taylor",
"PlayerScores" : [
7848474,
8746345353
]
}
{
"_id" : ObjectId("5cc6f531af8e7a4ca6b2ad9b"),
"PlayerName" : "David Miller",
"PlayerScores" : [ ]
}以下是查询以检查文档中的字段是否存在值。在这里，我们正在检查字段'PlayerScores，其值为[]-
> db.checkIfValueDemo.find({'PlayerScores.0' : {$exists: true}}).count();这将产生以下输出
2