I wrote a short article about query projection in DAX. To make the story short, you should always use SUMMARIZE in order to remove columns from a table in a query. Thus, if you just want three columns from a table, instead of writing this in SQL:
SELECT Col1, Col2, Col3 FROM Table
you should write this in DAX:
EVALUATE SUMMARIZE( Table, Table[Col1], Table[Col2], Table[Col3] )
A longer explanation is available in this article on SQLBI, that also explain how to rename a column, even if there are penalty performances doing that. Feedback are welcome in case you have suggestions!
Originally appeared on: http://sqlblog.com/blogs/marco_russo/archive/2011/12/29/query-projection-in-dax.aspx