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This article will tell you about all the fields in a database and how to check if they exist. In addition, you will learn how to check if an embedded field exists in a database.

Fields in the database

A field is a physical structure that holds data in a form, file, or database. A field contains one or more bytes.

Data records include order number, name, address, city, etc. The two words 搜索字段and 搜索框are often used interchangeably on web pages.

For database searches, this field is the common denominator. For example, when you run a database query to find the 居住在某个国家/地区的所有客户, the column is used STATE.

ORDER AMOUNTField is used to total when aggregating transactions. When looking for a specific employee, use JOB TITLE.

Fields in MongoDB

In MongoDB, each document stored in a collection requires a unique _idprimary key field. If the inserted document omits _idthe field, the MongoDB driver automatically _idgenerates one for the field ObjectId.

Check if field exists in MongoDB

$existsThe operator in MongoDB can be used to verify the existence of a field in a given collection. When $existsthe value of the operator is set to true, it matches documents that contain the provided field (including documents where the value of that field is empty).

When $existsthe value of the operator is false, this operator returns only documents that do not contain the given field.

grammar:

{ field: { $exists: <boolean> } }

When <boolean>is true, $existsmatches documents that contain the field, including nulldocuments where the field value is . If <boolean>is false, the query returns only documents that do not contain the field.

MongoDB $existsdoes not correspond to the SQL _ operator exists. For SQL _ exists, use $inthe _ operator.

$inThe operator finds documents where a field value is equal to any of the values ​​in the provided array. Use the following prototype to provide $inan expression.

{ field: { $in: [<value1>, <value2>, ... <valueN> ] } }

In MongoDB, you can use the following method to check if a field exists in a collection.

Check if a field exists in a MongoDB collection

db.data.find({ "myField": { $exists: true } })

This method determines myFieldif exists in datathe collection. If it does, it returns all documents that contain the field name. If not, it returns nothing.

example:

db={
  "data": [
    {
      "_id": ObjectId("611a99100a3322fc1bd8c38b"),
      "fname": "Tom",
      "city": "United States of America",
      "StudentHobby": [
        "c#",
        "asp",
        "gaming"
      ]
    },
    {
      "_id": ObjectId("611a99340a3322fc1bd8c38c"),
      "fname": "Harry",
      "city": "Canada",
      "courses": [
        "python",
        "asp",
        "node"
      ]
    },
    {
      "_id": ObjectId("611a99510a3322fc1bd8c38d"),
      "fname": "Mikky",
      "city": "New Zealand",
      "courses": [
        "python",
        "asp",
        "c++"
      ]
    },
    {
      "_id": ObjectId("611b3e88a60b5002406571c3"),
      "fname": "Ron",
      "city": "United Kingdom",
      "courses": [
        "python",
        "django",
        "node"
      ]
    }
  ]
}

The database configuration is as above; StudentHobbythe query to check whether the field exists is:

db.data.find({ 'StudentHobby' : { '$exists' : true }})

The results of this query are shown in the following screenshot.

Check if embedded field exists in MongoDB

db.data.find({ "myField.embeddedField": { $exists: true } })

This method determines whether myFieldthe fields in the field are present in the collection.embeddedFielddata

If yes, it returns all documents containing the field name. If no, it returns nothing.

The database configuration for the above query is:

db.teams.insertOne({team: "Mavs", class: {conf:"Western", div:"A"}, points: 31})
db.teams.insertOne({team: "Spurs", class: {conf:"Western", div:"A"}, points: 22})
db.teams.insertOne({team: "Jazz", class: {conf:"Western", div:"B"}, points: 19})
db.teams.insertOne({team: "Celtics", class: {conf:"Eastern", div:"C"}, points: 26})

The following code will show divwhether the embedded field name exists in teamsthe fields in collection class.

db.teams.find({ "class.div": { $exists: true } })

The results of this query are shown in the following screenshot.

Since the embedded field name divexists in classthe field, divevery document containing the embedded field is returned.

When a field does not exist in MongoDB

db={
  "data": [
    {
      "_id": ObjectId("611a99100a3322fc1bd8c38b"),
      "fname": "Tom",
      "city": "United States of America",
      "StudentHobby": [
        "c#",
        "asp",
        "gaming"
      ]
    },
    {
      "_id": ObjectId("611a99340a3322fc1bd8c38c"),
      "fname": "Harry",
      "city": "Canada",
      "courses": [
        "python",
        "asp",
        "node"
      ]
    },
    {
      "_id": ObjectId("611a99510a3322fc1bd8c38d"),
      "fname": "Mikky",
      "city": "New Zealand",
      "courses": [
        "python",
        "asp",
        "c++"
      ]
    },
    {
      "_id": ObjectId("611b3e88a60b5002406571c3"),
      "fname": "Ron",
      "city": "United Kingdom",
      "courses": [
        "python",
        "django",
        "node"
      ]
    }
  ]
}

If the field doesn't exist, you won't get anything.

db.data.find({ 'StudentTechnicalSubject' : { '$exists' : true }})

no document found

The query function of any database management system is crucial to obtain data. Since the databases of large enterprises contain complex data types, they prefer to use queries to access the required data in a timely manner.

Operators are an important part of every query; in this tutorial, you practiced $existsthe operator of MongoDB. You can use this operator to verify the available fields in the document and fetch papers that do not contain the requested fields.

$existsThe functionality of the operators mentioned above 布尔值is supported by which can be passed.