Group By Grouping Sets

CREATE TABLE employee
(
  name          NVARCHAR2(10),
  gender        NCHAR(1),
  country       NVARCHAR2(10),
  department    NVARCHAR2(10),
  salary        NUMBER(10)
);
INSERT INTO employee VALUES ('张三','男','中国','市场部',4000);
INSERT INTO employee VALUES ('李四','男','中国','市场部',5000);
INSERT INTO employee VALUES ('王五','女','美国','市场部',3000);  
INSERT INTO employee VALUES ('赵红','男','中国','技术部',2000);
INSERT INTO employee VALUES ('李白','女','中国','技术部',5000);  
INSERT INTO employee VALUES ('王蓝','男','美国','技术部',4000);
commit;

SELECT country, null department, round(avg(salary), 2) FROM employee GROUP BY country
UNION ALL
SELECT null country, department, round(avg(salary), 2) FROM employee GROUP BY department;
等价于
SELECT country, department, round(avg(salary), 2) FROM employee GROUP BY GROUPING SETS (country, department);


GROUP BY GROUPING SETS (A,B,C)  等价与  GROUP BY A  
                                        UNION ALL  
                                        GROUP BY B  
                                        UNION ALL  
                                        GROUP BY C
                                        
                                        
GROUP BY GROUPING SETS ((A,B,C))  等价与  GROUP BY A,B,C  
   
   
GROUP BY GROUPING SETS (A,(B,C))  等价与  GROUP BY A  
                                          UNION ALL  
                                          GROUP BY B,C

                                        
GROUP BY GROUPING SETS (A)  等价于  GROUP BY A,B,C  
        ,GROUPING SETS (B)  
        ,GROUPING SETS (C)  
   
   
GROUP BY GROUPING SETS (A)  等价于  GROUP BY A,B,C  
        ,GROUPING SETS ((B,C))   
   
   
GROUP BY GROUPING SETS (A)  等价于  GROUP BY A,B  
        ,GROUPING SETS (B,C)        UNION ALL  
                                    GROUP BY A,C
                                    
                                    
GROUP BY A                     等价于  GROUP BY A  
        ,B                                     ,B  
        ,GROUPING SETS ((B,C))                 ,C  
   
   
GROUP BY A                    等价于  GROUP BY A,B,C  
        ,B                            UNION ALL  
        ,GROUPING SETS (B,C)          GROUP BY A,B  
   
   
GROUP BY A                    等价于 GROUP BY A,B,C  
        ,B                           UNION ALL  
        ,C                           GROUP BY A,B,C  
        ,GROUPING SETS (B,C)