Читаем

Syntax
[ GROUP BY [ ALL ] group_by_expression [ ,...n ]
[ WITH { CUBE | ROLLUP } ]
]
...
group_by_expression

Is an expression on which grouping is performed. group_by_expression is also known as a grouping column. group_by expression can be a column or a nonaggregate expression that references a column. A column alias that is defined in the select list cannot be used to specify a grouping column.