How to select a single field for all documents in a MongoDB collection

Question

In my MongoDB  I have a student collection with 10 records having fields name and roll  One record of this collection is            id    ObjectId  53d9feff55d6b4dd1171dd9e         name     Swati        roll     80       I want to retrieve the field roll only for all 10 records in the collection as we would do in traditional database by using   SELECT roll FROM student   I went through many blogs but all are resulting in a query which must have WHERE clause in it  for example   db students find    roll      gt  70      The query is equivalent to   SELECT   FROM student WHERE roll  gt  70   My requirement is to find a single key only without any condition  So  what is the query operation for that

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This works for me   db student find      roll  1     no condition in where clause i e   inside first curly braces  inside next curly braces  list of projection field names to be needed in the result and 1 indicates particular field is the part of the query result

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Using Studio 3T for MongoDB  if I use  find        id  0  roll  true    it still return an array of objects with an empty  id property   Using JavaScript map helped me to only retrieve the desired roll property as an array of string   var rolls   db student    find   roll     gt  70         query where role  gt  70    map x   gt  x roll                return an array of role

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From the MongoDB docs   A projection can explicitly include several fields  In the following operation  find   method returns all documents that match the query  In the result set  only the item and qty fields and  by default  the  id field return in the matching documents  db inventory find    type   food       item  1  qty  1      In this example from the folks at Mongo  the returned documents will contain only the fields of item  qty  and  id   Thus  you should be able to issue a statement such as  db students find      roll 1   id 0    The above statement will select all documents in the students collection  and the returned document will return only the roll field  and exclude the  id   If we don t mention  id 0 the fields returned will be roll and  id  The   id  field is always displayed by default  So we need to explicitly mention  id 0 along with roll

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Not sure this answers the question but I believe it s worth mentioning here   There is one more way for selecting single field  and not multiple  using db collection name distinct     e g  db student distinct  roll        Or  2nd way  Using db collection name find   forEach     multiple fields can be selected here by concatenation   e g   db collection name find   forEach function c1  print c1 roll

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get all data from table db student find       SELECT   FROM student   get all data from table without  id db student find       id 0     SELECT name  roll FROM student   get all data from one field with  id db student find      roll 1     SELECT id  roll FROM student   get all data from one field without  id db student find      roll 1   id 0     SELECT roll FROM student   find specified data using where clause db student find  roll  80     SELECT   FROM students WHERE roll    80    find a data using where clause and greater than condition db student find    quot roll quot      gt  70         gt is greater than    SELECT   FROM student WHERE roll  gt   70    find a data using where clause and greater than or equal to condition db student find    quot roll quot      gte  70         gte is greater than or equal   SELECT   FROM student WHERE roll  gt    70    find a data using where clause and less than or equal to condition db student find    quot roll quot      lte  70         lte is less than or equal   SELECT   FROM student WHERE roll  lt    70    find a data using where clause and less than to condition db student find    quot roll quot      lt  70          lt is less than   SELECT   FROM student WHERE roll  lt   70

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Apart from what people have already mentioned I am just introducing indexes to the mix  So imagine a large collection  with let s say over 1 million documents and you have to run a query like this  The WiredTiger Internal cache will have to keep all that data in the cache if you have to run this query on it  if not that data will be fed into the WT Internal Cache either from FS Cache or Disk before the retrieval from DB is done  in batches if being called for from a driver connected to database  amp  given that 1 million documents are not returned in 1 go  cursor comes into play  Covered query can be an alternative  Copying the text from docs directly  When an index covers a query  MongoDB can both match the query conditions and return the results using only the index keys  i e  MongoDB does not need to examine documents from the collection to return the results  When an index covers a query  the explain result has an IXSCAN stage that is not a descendant of a FETCH stage  and in the executionStats  the totalDocsExamined is 0  Query    db getCollection  qaa   find  roll no     gte   0     id   0  roll no   1    Index   db getCollection  qaa   createIndex  roll no   1    If the index here is in WT Internal Cache then it would be a straight forward process to get the values  An index has impact on the write performance of the system thus this would make more sense if the reads are a plenty compared to the writes

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In mongodb 3 4 we can use below logic  i am not sure about previous versions  select roll from student      db student find       roll 1    the above logic helps to define some columns  if they are less

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Just for educational purposes you could also do it with any of the following ways   1       var query     roll     gt  70       var cursor   db student find query       cursor project   roll  1    id  0      2       var query     roll     gt  70       var projection     roll  1    id  0       var cursor   db student find query projection

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id    quot 123321 quot    user     await likes find  liker id   id   liked id  quot  liked id quot        let suppose you have liker id and liked id field in the document so by putting  quot  liked id quot  it will return  id and liked id only

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If u want to retrieve the field  roll  only for all 10 records in the collections  Then try this   In MongoDb        db students find            roll          roll        In Sql            select roll from students

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getting name of the student   student-details   db students find     roll     gt  70      name   1    id   False        getting name  amp  roll of the student   student-details   db students find     roll     gt  70     name   1  roll  1   id   False

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Use the Query like this in the shell   1  Use database name  e g  use database name   2  Which returns only assets particular field information when matched    id 0 specifies not to display ID in the result  db collection name find     Search Field    value                           Field to display   1  id 0

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The query for MongoDB here fees is collection and description is a field   db getCollection  fees   find     description 1  id 0

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I just want to add to the answers that if you want to display a field that is nested in another object  you can use the following syntax  db collection find          object key   true     Here key is present inside the object named object      id    ObjectId  5d2ef0702385     object       key     value

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db student find       roll  1    id  0     This is equivalent to -     Select roll from student        db student find       roll  1   name  1    id  0    This is equivalent to -     Select roll  name from student

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For better understanding I have written similar MySQL query   Selecting specific fields       MongoDB   db collection name find     name true email true phone true         MySQL   SELECT name email phone FROM table name    Selecting specific fields with where clause      MongoDB   db collection name find  email  you email com    name true email true phone true         MySQL   SELECT name email phone FROM table name WHERE email    you email com

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I think mattingly890 has the correct answer   here is another example along with the pattern commmand  db collection find        your key 1   id 0

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var collection   db collection  appuser        collection aggregate           project     firstName   1  lastName   1     function err  res           res toArray function err  realRes             console log  quot response roo   gt  quot  realRes                             it s working

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While gowtham s answer is complete  it is worth noting that those commands may differ from on API to another  for those not using mongo s shell   Please refer to documentation link for detailed info   Nodejs  for instance  have a method called  projection that you would append to your find function in order to project   Following the same example set  commands like the following can be used with Node   db student find     project  roll 1       SELECT  id  roll FROM student   Or db student find     project  roll 1   id  0       SELECT roll FROM student   and so on   Again for nodejs users  do not forget  what you should already be familiar with if you used this API before  to use toArray in order to append your  then command

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db  lt collection gt  find      field1   lt value gt   field2   lt value gt          In your example  you can do something like   db students find       roll  true    id  false     Projection     The projection parameter determines which fields are returned in the   matching documents  The projection parameter takes a document of the   following form      field1   lt value gt   field2   lt value gt           The  lt value gt  can be any of the following           1 or true to include the field in the return documents    0 or false to exclude the field       NOTE     For the  id field  you do not have to explicitly specify  id  1 to   return the  id field  The find   method always returns the  id field   unless you specify  id  0 to suppress the field    READ MORE

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Try the following query   db student find      roll  1   id  0   pretty      Hope this helps

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Here you go   3 ways of doing   Shortest to boring    db student find      roll  id        lt --- Just multiple fields name space separated    OR db student find     select  roll  id        lt --- Just multiple fields name space separated    OR db student find       roll    1     id    1        lt ---- Old lengthy boring way   To remove specific field use - operator    db student find     select  roll - id       lt --- Will remove id from result

[mongodb] How to select a single field for all documents in a MongoDB collection?

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