On a picture below you may see how the columns are positioned on a table.

 

 

It is allowed to specify one or several columns at once. For example, in cross table only one column is specified:

 

 

As a result we get grouping by values of this column:

 

 

If to specify more than one column:

 

 

Grouping is output by values of two columns. Values of the first column are output first. Then the value from the second column is output: