MongoDB Database, equivalent for SELECT column1, c

问题:

From my MongoDB I want the equivalent for

  SELECT column1, column2
  FROM tbl

With this code I get all the 'rows' but also all the 'columns'

  DBCollection collection = database.getCollection("names");
  DBCursor cursor = collection.find();

I for example would like all the 'rows' but only the 'columns': id, name, age

How would I do this?

Thanks for any help!!

回答1:

db.collection.find({}, {_id: 1, name: 1, age: 1})

The first argument to find (the predicate) is your selection criteria, e.g.

db.collection.find({age: {$gte: 21}})

The second limits the fields you retrieve, so for the names over everyone 21 or older:

db.collection.find({age: {$gte: 21}}, {name: 1})

The field selector always pulls back _id unless you specifically turn it off:

db.collection.find({}, {_id: 0})

However, Mongo will not check for field existence by default. If you want to select certain fields, and match only results that have those fields, you will want to use:

db.collection.find({ age: { $exists: true } })

The MongoDB website has a more detailed description of the .find() function!

回答2:

Thanks!, I fixed it with the code below.

    BasicDBObject select = new BasicDBObject();
    select.put("id", 1);
    select.put("name", 1);
    select.put("age", 1);
    collection.find(new BasicDBObject(), select);

Above code gives me all the records, with only the column names as above.

回答3:

If you want select one or multiple columns like SQL query. for example if your SQL query like this

select name, content from knowledgebase where applicationId='2955f3e174dce55190a87ed0e133adwdeb92';

MongoDB query:

db.knowledgebase.find({ "applicationId": "2955f3e174dce55190a87ed0e133adwdeb92"}, { "name": 1,    "content": 1});