MongoDB : Select/Query/Find Document(s) Examples

In previous tutorials, We have learned about MongoDB basics and installing MongoDB in Windows. Now in this tutorial, we will learn about a number of ways to select or query either a particular single document or multiple documents based on same condition as we do in SQL using WHERE CLAUSE.

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Query Examples covered in this tutorial

1) Select all documents from a collection
2) Select first document from a collection
3) Select single document and limited field(s) from a collection
4) Select all documents with unique id from a collection
5) Document ids IN clause example
6) Document ids less than or greater than clause example
7) Document ids NOT IN clause example
8) Documents matching multiple fields example
9) Documents matching field value to some REGEX example

Test data preparation

For building and running examples used in this tutorial, I have populated 20 employee documents in database with employeeId from 1 to 20, and employeeName from TestEmployee_1 to TestEmployee_10.

private static void setUpTestData(DBCollection collection){
    for (int i=1; i <= 10; i++) {
        collection.insert(new BasicDBObject().append("employeeId", i).append("employeeName", "TestEmployee_"+i));
    }
}

Now run our examples and fetch the data on different scenarios.

1) Select all documents from a collection

private static void selectAllRecordsFromACollection(DBCollection collection) {
    DBCursor cursor = collection.find();
    while(cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}

Output:

{ "_id" : { "$oid" : "538782753641d31b0cad0142"} , "employeeId" : 1 , "employeeName" : "TestEmployee_1"}
{ "_id" : { "$oid" : "538782753641d31b0cad0143"} , "employeeId" : 2 , "employeeName" : "TestEmployee_2"}
{ "_id" : { "$oid" : "538782753641d31b0cad0144"} , "employeeId" : 3 , "employeeName" : "TestEmployee_3"}
{ "_id" : { "$oid" : "538782753641d31b0cad0145"} , "employeeId" : 4 , "employeeName" : "TestEmployee_4"}
{ "_id" : { "$oid" : "538782753641d31b0cad0146"} , "employeeId" : 5 , "employeeName" : "TestEmployee_5"}
{ "_id" : { "$oid" : "538782753641d31b0cad0147"} , "employeeId" : 6 , "employeeName" : "TestEmployee_6"}
{ "_id" : { "$oid" : "538782753641d31b0cad0148"} , "employeeId" : 7 , "employeeName" : "TestEmployee_7"}
{ "_id" : { "$oid" : "538782753641d31b0cad0149"} , "employeeId" : 8 , "employeeName" : "TestEmployee_8"}
{ "_id" : { "$oid" : "538782753641d31b0cad014a"} , "employeeId" : 9 , "employeeName" : "TestEmployee_9"}
{ "_id" : { "$oid" : "538782753641d31b0cad014b"} , "employeeId" : 10 , "employeeName" : "TestEmployee_10"}

2) Select first document from a collection

private static void selectFirstRecordInCollection(DBCollection collection) {
    DBObject dbObject = collection.findOne();
    System.out.println(dbObject);
}

Output:

{ "_id" : { "$oid" : "538782a53641ed9125df86c0"} , "employeeId" : 1 , "employeeName" : "TestEmployee_1"}

3) Select single document and limited field(s) from a collection

private static void selectSingleRecordAndFieldByRecordNumber(DBCollection collection) {
    BasicDBObject whereQuery = new BasicDBObject();
    whereQuery.put("employeeId", 5);
    BasicDBObject fields = new BasicDBObject();
    fields.put("employeeId", 1);
 
    DBCursor cursor = collection.find(whereQuery, fields);
    while (cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}
    
Output:

{ "_id" : { "$oid" : "53878332364101041fb2c141"} , "employeeId" : 5}

4) Select all documents with unique id from a collection

private static void selectAllRecordByRecordNumber(DBCollection collection) {
    BasicDBObject whereQuery = new BasicDBObject();
    whereQuery.put("employeeId", 5);
    DBCursor cursor = collection.find(whereQuery);
    while(cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}
Output:

{ "_id" : { "$oid" : "538783623641e9b2da299fa7"} , "employeeId" : 5 , "employeeName" : "TestEmployee_5"}

5) Document ids IN clause example

private static void in_Example(DBCollection collection) {
    BasicDBObject inQuery = new BasicDBObject();
    List<Integer> list = new ArrayList<Integer>();
    list.add(2);
    list.add(4);
    list.add(5);
    inQuery.put("employeeId", new BasicDBObject("$in", list));
    DBCursor cursor = collection.find(inQuery);
    while(cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}

Output:

{ "_id" : { "$oid" : "5387838d3641024de174c795"} , "employeeId" : 2 , "employeeName" : "TestEmployee_2"}
{ "_id" : { "$oid" : "5387838d3641024de174c797"} , "employeeId" : 4 , "employeeName" : "TestEmployee_4"}
{ "_id" : { "$oid" : "5387838d3641024de174c798"} , "employeeId" : 5 , "employeeName" : "TestEmployee_5"}

6) Document ids less than or greater than clause example

private static void lessThan_GreaterThan_Example(DBCollection collection) {
    BasicDBObject getQuery = new BasicDBObject();
    getQuery.put("employeeId", new BasicDBObject("$gt", 2).append("$lt", 5));
    DBCursor cursor = collection.find(getQuery);
    while(cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}
    
Output:

{ "_id" : { "$oid" : "538783b63641720cd34f98c8"} , "employeeId" : 3 , "employeeName" : "TestEmployee_3"}
{ "_id" : { "$oid" : "538783b63641720cd34f98c9"} , "employeeId" : 4 , "employeeName" : "TestEmployee_4"}

7) Document ids NOT IN clause example

private static void negation_Example(DBCollection collection) {
    BasicDBObject neQuery = new BasicDBObject();
    neQuery.put("employeeId", new BasicDBObject("$ne", 4));
    DBCursor cursor = collection.find(neQuery);
    while(cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}
Output:

{ "_id" : { "$oid" : "538783db3641d3ca9d400510"} , "employeeId" : 1 , "employeeName" : "TestEmployee_1"}
{ "_id" : { "$oid" : "538783db3641d3ca9d400511"} , "employeeId" : 2 , "employeeName" : "TestEmployee_2"}
{ "_id" : { "$oid" : "538783db3641d3ca9d400512"} , "employeeId" : 3 , "employeeName" : "TestEmployee_3"} 
{ "_id" : { "$oid" : "538783db3641d3ca9d400514"} , "employeeId" : 5 , "employeeName" : "TestEmployee_5"} //4 is missing
{ "_id" : { "$oid" : "538783db3641d3ca9d400515"} , "employeeId" : 6 , "employeeName" : "TestEmployee_6"}
{ "_id" : { "$oid" : "538783db3641d3ca9d400516"} , "employeeId" : 7 , "employeeName" : "TestEmployee_7"}
{ "_id" : { "$oid" : "538783db3641d3ca9d400517"} , "employeeId" : 8 , "employeeName" : "TestEmployee_8"}
{ "_id" : { "$oid" : "538783db3641d3ca9d400518"} , "employeeId" : 9 , "employeeName" : "TestEmployee_9"}
{ "_id" : { "$oid" : "538783db3641d3ca9d400519"} , "employeeId" : 10 , "employeeName" : "TestEmployee_10"}

8) Documents matching multiple fields example

private static void andLogicalComparison_Example(DBCollection collection) {
    BasicDBObject andQuery = new BasicDBObject();
    List<BasicDBObject> obj = new ArrayList<BasicDBObject>();
    obj.add(new BasicDBObject("employeeId", 2));
    obj.add(new BasicDBObject("employeeName", "TestEmployee_2"));
    andQuery.put("$and", obj);
 
    System.out.println(andQuery.toString());
 
    DBCursor cursor = collection.find(andQuery);
    while (cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}

Output:

{ "$and" : [ { "employeeId" : 2} , { "employeeName" : "TestEmployee_2"}]}
{ "_id" : { "$oid" : "5387840336418a41167caaa4"} , "employeeId" : 2 , "employeeName" : "TestEmployee_2"}

9) Documents matching field value to some REGEX example

private static void regex_Example(DBCollection collection) {
    BasicDBObject regexQuery = new BasicDBObject();
    regexQuery.put("employeeName", 
        new BasicDBObject("$regex", "TestEmployee_[3]")
        .append("$options", "i"));
 
    System.out.println(regexQuery.toString());
 
    DBCursor cursor = collection.find(regexQuery);
    while (cursor.hasNext()) {
        System.out.println(cursor.next());
    }
}

Output:

{ "employeeName" : { "$regex" : "TestEmployee_[3]" , "$options" : "i"}}
{ "_id" : { "$oid" : "538784213641917ce7068c57"} , "employeeId" : 3 , "employeeName" : "TestEmployee_3"}

Complete Code for all examples

package examples.mongodb.crud;

import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.List;

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 com.mongodb.WriteResult;

public class MongoDBSelectExample {
    
    private static void setUpTestData(DBCollection collection){
        for (int i=1; i <= 10; i++) {
            collection.insert(new BasicDBObject().append("employeeId", i).append("employeeName", "TestEmployee_"+i));
        }
    }
    
    public static void main(String[] args) throws UnknownHostException 
    {
        MongoClient mongo = new MongoClient("localhost", 27017);
        DB db = mongo.getDB("howtodoinjava");
        DBCollection collection = db.getCollection("users");
        
        //Delete All documents before running example again
        WriteResult result = collection.remove(new BasicDBObject());
        System.out.println(result.toString());
        
        //Set up test data
        setUpTestData(collection);
        
        //Select all document from a collection
        selectAllRecordsFromACollection(collection);
        
        //Select first document in collection
        selectFirstRecordInCollection(collection);
        
        //Select single document and single field based on record number
        selectSingleRecordAndFieldByRecordNumber(collection);
        
        //Select all documents where record number = n
        selectAllRecordByRecordNumber(collection);
        
        //In example
        in_Example(collection);
        
        //Less than OR greater than example
        lessThan_GreaterThan_Example(collection);
        
        //Select document where record number != n
        negation_Example(collection);
        
        //And logical comparison query example
        andLogicalComparison_Example(collection);
        
        //Select documents based on regex match LIKE example
        regex_Example(collection);
        
    }

    private static void selectFirstRecordInCollection(DBCollection collection) {
        DBObject dbObject = collection.findOne();
        System.out.println(dbObject);
    }

    private static void selectAllRecordsFromACollection(DBCollection collection) {
        DBCursor cursor = collection.find();
        while(cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void selectSingleRecordAndFieldByRecordNumber(DBCollection collection) {
        BasicDBObject whereQuery = new BasicDBObject();
        whereQuery.put("employeeId", 5);
        BasicDBObject fields = new BasicDBObject();
        fields.put("employeeId", 1);
     
        DBCursor cursor = collection.find(whereQuery, fields);
        while (cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void selectAllRecordByRecordNumber(DBCollection collection) {
        BasicDBObject whereQuery = new BasicDBObject();
        whereQuery.put("employeeId", 5);
        DBCursor cursor = collection.find(whereQuery);
        while(cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void in_Example(DBCollection collection) {
        BasicDBObject inQuery = new BasicDBObject();
        List<Integer> list = new ArrayList<Integer>();
        list.add(2);
        list.add(4);
        list.add(5);
        inQuery.put("employeeId", new BasicDBObject("$in", list));
        DBCursor cursor = collection.find(inQuery);
        while(cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void lessThan_GreaterThan_Example(
            DBCollection collection) {
        BasicDBObject gtQuery = new BasicDBObject();
        gtQuery.put("employeeId", new BasicDBObject("$gt", 2).append("$lt", 5));
        DBCursor cursor = collection.find(gtQuery);
        while(cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void negation_Example(DBCollection collection) {
        BasicDBObject neQuery = new BasicDBObject();
        neQuery.put("employeeId", new BasicDBObject("$ne", 4));
        DBCursor cursor = collection.find(neQuery);
        while(cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void andLogicalComparison_Example(DBCollection collection) {
        BasicDBObject andQuery = new BasicDBObject();
        List<BasicDBObject> obj = new ArrayList<BasicDBObject>();
        obj.add(new BasicDBObject("employeeId", 2));
        obj.add(new BasicDBObject("employeeName", "TestEmployee_2"));
        andQuery.put("$and", obj);
     
        System.out.println(andQuery.toString());
     
        DBCursor cursor = collection.find(andQuery);
        while (cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }

    private static void regex_Example(DBCollection collection) {
        BasicDBObject regexQuery = new BasicDBObject();
        regexQuery.put("employeeName", 
            new BasicDBObject("$regex", "TestEmployee_[3]")
            .append("$options", "i"));
     
        System.out.println(regexQuery.toString());
     
        DBCursor cursor = collection.find(regexQuery);
        while (cursor.hasNext()) {
            System.out.println(cursor.next());
        }
    }
}

That’s all for different techniques to fetch data from MongoDB. In next post, I will cover about inserting documents in MongoDB.

Happy Learning !!

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3 thoughts on “MongoDB : Select/Query/Find Document(s) Examples”

  1. Thanks for your wonderful post.
    I have one query
    DB db = mongo.getDB(“howtodoinjava”);
    DBCollection collection = db.getCollection(“users”);

    Here we are getting the ‘howtodoinjava’ and ‘users’. But where are we actually setting them.
    Might look like a dumb question. But still need help :)

    Thanks again. Keep Posting!!!

    1. Collections in MongoDB are created lazily as soon as they are written to. Once you insert a document, the collection will automatically be created.

  2. Nice post Lokesh, I have seen that annotations can be used to map POJO’s and documents from collections in MongoDB
    (in Spring), would be great if you take a look into it and share your experience!

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