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Using Select in MongoDB

MongoDB is a cross-platform, document oriented database that provides, high performance, high availability, and easy scalability. MongoDB works on concept of collection and document.

MongoDB as-a-Service

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"}

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"}

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}

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"}
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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"}

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"}

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"}

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"}

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.

For indepth understanding click on

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Using Select in MongoDB
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