A quick brain twist from SQL to MongoDB

Underlying features of SQL work differently in MongoDB, but this chart can give you a quick boost on how the commands translate:
http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/

SQL Terms, Functions, and Concepts	MongoDB Aggregation Operators
WHERE	$match
GROUP BY	$group
HAVING	$match
SELECT	$project
ORDER BY	$sort
LIMIT	$limit
SUM()	$sum
COUNT()	$sum
join	No direct corresponding operator; however, the $unwind operator allows for somewhat similar functionality, but with fields embedded within the document.

Head to the page to see examples.