Q:

How to translate this SQL into mongodb? --  adding 2 strings

How to translate this SQL: 
Select fullName = firstName + ' ' + lastName from People

into MongoDB's query? 
db.People.find(???)  

db.People.aggregate(???) 

here is the people collection
/* 0 */
{
  "_id" : ObjectId("56215bef66a6d31bdccc717e"),
  "firstName" : "John",
  "lastName" : "White"
}

/* 1 */
{
  "_id" : ObjectId("56215c1e66a6d31bdccc717f"),
  "firstName" : "Paul",
  "lastName" : "Smith"
}

A:

You can try to use the $concat operator.  Below is an example which might give you some idea. 
db.People.aggregate(
   [
      { $project: { fullName: { $concat: [ "$firstName", " ", "$lastName" ] } } }
   ]
)

