单级别聚合

最简单的 GROUP BY 查询是在单级别聚合数据。例如，表可能包含下列家庭开支信息：

• 类别

• 当年在该类别上花费的金额

• 年

表数据可能类似于以下内容：

=> SELECT * FROM expenses ORDER BY Category;
 Year |  Category  | Amount
------+------------+--------
2005  | Books      |  39.98
2007  | Books      |  29.99
2008  | Books      |  29.99
2006  | Electrical | 109.99
2005  | Electrical | 109.99
2007  | Electrical | 229.98

您可以使用聚合函数，获取每个类别或每年的总开支：

=> SELECT SUM(Amount), Category FROM expenses GROUP BY Category;
 SUM     | Category
---------+------------
  99.96  | Books
 449.96  | Electrical

=> SELECT SUM(Amount), Year FROM expenses GROUP BY Year;
 SUM    | Year
--------+------
 149.97 | 2005
 109.99 | 2006
  29.99 | 2008
 259.97 | 2007