# ROLLUP
#### 功能描述
ROLLUP生成聚合行、超聚合行和总计行。可以实现从右到左递减多级的统计，显示统计某一层次结构的聚合。
#### 语法格式
```
SELECT attr_expr_list FROM table_reference
  GROUP BY col_name_list
  WITH ROLLUP;
```
#### 关键字
ROLLUP：为GROUP BY的扩展，例如：***SELECT a, b, c, SUM(expression) FROM table GROUP BY a, b, c WITH ROLLUP;*** 将转换成以下四条查询：
- (a, b, c)组合小计
  ```
  SELECT a, b, c, sum(expression) FROM table
    GROUP BY a, b, c;
  ```
  
- (a, b)组合小计
  ```
  SELECT a, b, NULL, sum(expression) FROM table
    GROUP BY a, b;
  ```
  
- (a)组合小计
  ```
  SELECT a, NULL, NULL, sum(expression) FROM table
    GROUP BY a;
  ```
  
- 总计
  ```
  SELECT NULL, NULL, NULL, sum(expression) FROM table;
  ```
  
 
#### 注意事项
所要分组的表必须是已经存在的表，否则会出错。
#### 示例
根据group_id与job两个字段生成聚合行、超聚合行和总计行，返回每种聚合情况下的salary总和。
```
SELECT group_id, job, SUM(salary) FROM group_test
  GROUP BY group_id, job
  WITH ROLLUP;
```