This checks the document for the existence of the fields in the specified collection.

这将检查文档中指定集合中是否存在字段。

The syntax is

语法是

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

The operator accepts the boolean values of either true or false.

运算符接受布尔值true或false。

If the boolean value is set to true, the exists operator matches the documents that contain the fields specified in the input parameters. If the boolean option is set to false the query returns the documents which do not contain the field.

如果布尔值设置为true，则exist操作符将匹配包含输入参数中指定的字段的文档。 如果布尔选项设置为false，则查询将返回不包含该字段的文档。

Let’s check out the examples of usage of exists operator.

我们来看一下存在运算符的用法示例。

存在运算符设置为true (Exists operator set to true)

This operation returns only the documents that contain the field specified in the query.

此操作仅返回包含查询中指定的字段的文档。

>db.car.find({ regno:{ $exists:true}})
{ "_id" : 51, "name" : "NissanSunny", "cno" : "H678", "regno" : 141, "speed" : 25 }
{ "_id" : 52, "name" : "Fiat", "cno" : "H679", "regno" : 142, "speed" : 57 }

The documents having the regno field are fetched when exists is set to true.

当存在设置为true时，将获取具有regno字段的文档。

将运算符设置为true，并指定选择标准 (Exists operator set to true and selection criteria specified)

This operation returns only the documents which satisfy the criteria entered and contain the fields specified in the query.

此操作仅返回满足输入条件并包含查询中指定字段的文档。

>db.car.find({ speed:{ $exists:true , $gt:80 }}){ "_id" : ObjectId("5474896b93f400069d439c04"), "name" : "Micra", "color" : "Lime", "cno" : "H186", "mfdcountry" : "Ethopia", "speed" : 84 }
{ "_id" : ObjectId("5474896b93f400069d439c03"), "name" : "Palio", "color" : "Purple", "cno" : "H183", "mfdcountry" : "Venice", "speed" : 82 }

The documents having the speed field and the speed greater than 80 are retrieved from the collection.

从集合中检索速度字段和速度大于80的文档。

存在运算符设置为false (Exists operator set to false)

This retrieves the documents that do not contain the fields specified in the query.

这将检索不包含查询中指定的字段的文档。

>db.car.find( { mfdcountry: { $exists:false}}){ "_id" : 8, "name" : "Zen", "speed" : 54 }
{ "_id" : ObjectId("5474896b93f400069d439c00"), "name" : "Indica", "color" : "Silver", "cno" : "H154" }
{ "_id" : 43, "name" : "Astar", "speed" : 79 }
{ "_id" : 51, "name" : "NissanSunny", "cno" : "H678", "regno" : 141, "speed" : 25 }
{ "_id" : 52, "name" : "Fiat", "cno" : "H679", "regno" : 142, "speed" : 57 }
{ "_id" : 59, "name" : "Quanta-45", "cno" : null, "regno" : null, "speed" : null }
{ "_id" : 99, "name" : "Brio", "cno" : null, "regno" : null, "speed" : null }

检索具有空值的文档 (Retrieve documents having null values)

This retrieves the documents that contains the null values for the field specified in the query.

这将检索包含查询中指定字段的空值的文档。

>db.car.find( { speed: { $exists:true } }){ "_id" : 8, "name" : "Zen", "speed" : 54 }
{ "_id" : 6, "name" : "HondaCity", "color" : "Grey", "cno" : "H106", "mfdcountry" : "Sweden", "speed" : 45 }
{ "_id" : ObjectId("5474642dd785e3a05a1808a7"), "name" : "Punto", "color" : "Wine Red", "cno" : "H109", "mfdcountry" : "Paris", "speed" : 45 }
{ "_id" : 9, "name" : "SwiftDezire", "color" : "Maroon", "cno" : "H108", "mfdcountry" : "New York", "speed" : 40 }
{ "_id" : 51, "name" : "NissanSunny", "cno" : "H678", "regno" : 141, "speed" : 25 }
{ "_id" : 99, "name" : "Brio", "cno" : null, "regno" : null, "speed" : null }
{ "_id" : 59, "name" : "Quanta-45", "cno" : null, "regno" : null, "speed" : null }

This operation retrieves the document having null values for speed field.

此操作检索速度字段具有空值的文档。

适用于现有操作的MongoDB Java程序 (MongoDB Java Program for exists operations)

Let’s now write a java program to check whether the fields exists in the MongoDB.

现在，让我们编写一个Java程序来检查MongoDB中是否存在这些字段。

package com.journaldev.mongodb;import com.mongodb.BasicDBObject;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.DBCursor;
import com.mongodb.DBObject;
import com.mongodb.MongoClient;
import java.net.UnknownHostException;public class MongoDBExists {// method to check whether the field existspublic static void existstrue() throws UnknownHostException {// Get a db connectionMongoClient m1 = new MongoClient("localhost");// connect to test db,use your own hereDB db = m1.getDB("test");// obtain the car collectionDBCollection coll = db.getCollection("car");// checking whether regno field exists by setting exists to trueDBObject query = new BasicDBObject("regno", new BasicDBObject("$exists", true));// store the documents in cursor carDBCursor car = coll.find(query);// iterate and print the contents of cursortry {while (car.hasNext()) {System.out.println(car.next());}} finally {// close the cursorcar.close();}}// method to check the fields along along with the criteria enteredpublic static void existstruewithcriteria() throws UnknownHostException {MongoClient m1 = new MongoClient("localhost");DB db = m1.getDB("test");DBCollection coll = db.getCollection("car");// check whether speed fields exists and speed greater than 80DBObject query = new BasicDBObject("speed", new BasicDBObject("$exists", true).append("$gt", 80));DBCursor car = coll.find(query);System.out.println("-----------------------------------");try {while (car.hasNext()) {System.out.println(car.next());}} finally {car.close();}}// method to check the field does not existpublic static void existsfalse() throws UnknownHostException {MongoClient m1 = new MongoClient("localhost");DB db = m1.getDB("test");DBCollection coll = db.getCollection("car");// checking for mfdcountry field by setting exists to falseDBObject query = new BasicDBObject("mfdcountry", new BasicDBObject("$exists", false));DBCursor car = coll.find(query);System.out.println("-------------------------------------------------");try {while (car.hasNext()) {System.out.println(car.next());}} finally {car.close();}}public static void existstruewithnullvalues() throws UnknownHostException {MongoClient m1 = new MongoClient("localhost");DB db = m1.getDB("test");DBCollection coll = db.getCollection("car");// returns documents with null valuesDBObject query = new BasicDBObject("regno", new BasicDBObject("$exists", true));DBCursor car = coll.find(query);System.out.println("---------------------------------------------");try {while (car.hasNext()) {System.out.println(car.next());}} finally {car.close();}}public static void main(String[] args) throws UnknownHostException {// invoking all the methodsexiststrue();existstruewithcriteria();existsfalse();existstruewithnullvalues();}}

Output of the above program is:

上面程序的输出是：

{ "_id" : 51.0 , "name" : "NissanSunny" , "cno" : "H678" , "regno" : 141.0 , "speed" : 25.0}
{ "_id" : 52.0 , "name" : "Fiat" , "cno" : "H679" , "regno" : 142.0 , "speed" : 57.0}
--------------------------------------------------------------------------------------------------------------
{ "_id" : { "$oid" : "5474896b93f400069d439c04"} , "name" : "Micra" , "color" : "Lime" , "cno" : "H186" , "mfdcountry" : "Ethopia" , "speed" : 84}
{ "_id" : { "$oid" : "5474896b93f400069d439c03"} , "name" : "Palio" , "color" : "Purple" , "cno" : "H183" , "mfdcountry" : "Venice" , "speed" : 82}
------------------------------------------------------------------------------------------------------------------
{ "_id" : 8.0 , "name" : "Zen" , "speed" : 54.0}
{ "_id" : { "$oid" : "5474896b93f400069d439c00"} , "name" : "Indica" , "color" : "Silver" , "cno" : "H154"}
{ "_id" : 43 , "name" : "Astar" , "speed" : 79}
{ "_id" : 51.0 , "name" : "NissanSunny" , "cno" : "H678" , "regno" : 141.0 , "speed" : 25.0}
{ "_id" : 52.0 , "name" : "Fiat" , "cno" : "H679" , "regno" : 142.0 , "speed" : 57.0}
---------------------------------------------------------------------------------------------------------------
{ "_id" : 51.0 , "name" : "NissanSunny" , "cno" : "H678" , "regno" : 141.0 , "speed" : 25.0}
{ "_id" : 52.0 , "name" : "Fiat" , "cno" : "H679" , "regno" : 142.0 , "speed" : 57.0}
{ "_id" : 59.0 , "name" : "Quanta-45" , "cno" :  null  , "regno" :  null  , "speed" :  null }
{ "_id" : 99.0 , "name" : "Brio" , "cno" :  null  , "regno" :  null  , "speed" :  null }

MongoDB exists is a simple operation to understand and it can help us in selecting only specific documents that have the given field.

MongoDB存在是一个易于理解的操作，它可以帮助我们选择具有给定字段的特定文档。

翻译自: https://www.journaldev.com/6330/mongodb-exists-example-mongo-shell-java-driver