在047考题中有以下这么一道考题
11.View the Exhibit and examine the descriptions of ORDER_ITEMS and ORDERS tables.
   You want to display the CUSTOMER_ID, PRODUCT_ID, and total (UNIT_PRICE multiplied by
   QUANTITY) for the order placed. You also want to display the subtotals for a CUSTOMER_ID as well as
   for a PRODUCT_ID for the last six months.
   Which SQL statement would you execute to get the desired output?
A. SELECT o.customer_id, oi.product_id, SUM(oi.unit_price*oi.quantity) "Total"
   FROM order_items oi JOIN orders o
   ON oi.order_id=o.order_id GROUP BY ROLLUP (o.customer_id,oi.product_id) WHERE MONTHS_BETWEEN(order_date, SYSDATE) <= 6;
B. SELECT o.customer_id, oi.product_id, SUM(oi.unit_price*oi.quantity) "Total"
   FROM order_items oi JOIN orders o
   ON oi.order_id=o.order_id GROUP BY ROLLUP (o.customer_id,oi.product_id) HAVING MONTHS_BETWEEN(order_date, SYSDATE) <= 6;
C. SELECT o.customer_id, oi.product_id, SUM(oi.unit_price*oi.quantity) "Total"
   FROM order_items oi JOIN orders o
   ON oi.order_id=o.order_id GROUP BY ROLLUP (o.customer_id, oi.product_id) WHERE MONTHS_BETWEEN(order_date, SYSDATE) >= 6;
D. SELECT o.customer_id, oi.product_id, SUM(oi.unit_price*oi.quantity) "Total"
   FROM order_items oi JOIN orders o
   ON oi.order_id=o.order_id WHERE MONTHS_BETWEEN(order_date, SYSDATE) <= 6 GROUP BY ROLLUP (o.customer_id, oi.product_id) ;

Answer: D
上面这道题中其实要选择正确的答案是很简单的，就是group by应该放在where的后面，但是题目中出现的一个知识点需要我们注意，就是rollup的使用。
大家都熟悉group by基本语法，例如要统计每个部门的员工最高的工资，就可以使用
select max(sal) from emp group by deptno;
但是如果出现比较复杂的统计，单单靠group by基本用法是不能满足的需求的，还得配合rollup一起来使用，例如：
需要安装职位，经理，部门来统计员工的总工资，并且对职位，经理做一个子统计
SQL> select job,mgr,deptno,sum(sal)  from emp where deptno in(10,30)  group by rollup(job,mgr,deptno);
JOB                                MGR     DEPTNO   SUM(SAL)
--------------------------- ---------- ---------- ----------
CLERK                             7698         30        950
CLERK                             7698                   950
CLERK                             7782         10       1300
CLERK                             7782                  1300
CLERK                                                   2250
MANAGER                           7839         10       2450
MANAGER                           7839         30       2850
MANAGER                           7839                  5300
MANAGER                                                 5300
SALESMAN                          7698         30       5600
SALESMAN                          7698                  5600
JOB                                MGR     DEPTNO   SUM(SAL)
--------------------------- ---------- ---------- ----------
SALESMAN                                                5600
PRESIDENT                                      10       5000
PRESIDENT                                               5000
PRESIDENT                                               5000
                                                       18150
16 rows selected.
那么group by rollup的分组是按照以下步骤，在上述ROLLUP(job,mgr,deptno)例子中，首先会对(job,mgr,deptno)进行GROUP BY，然后对(job,mgr)进行GROUP BY，然后是(job)进行GROUP BY，最后对全表进行GROUP BY操作，结合上述的例子
首先对(job,mgr,deptno)分组，其统计结果类似如下：
CLERK                             7698         30        950
然后对(job,mgr)分组，其统计结果类似如下：
CLERK                             7698                   950
然后对(job)分组，其统计结果类似如下：
CLERK                                                   2250
最后对全表做一次分组统计
                                                       18150
这样分析以下，相信大家对group by rollup查询结构有一个全面的了解


CUUG

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