Single-level aggregation

The simplest GROUP BY queries aggregate data at a single level. For example, a table might contain the following information about family expenses:

• Category

• Amount spent on that category during the year

• Year

Table data might look like this:

=> 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

You can use aggregate functions to get the total expenses per category or per year:

=> 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