Union和Union All的区别
假设我们有一个表Student，包括以下字段与数据：

drop table student;

create table student
(
id int primary key,
name nvarchar2(50) not null,
score number not null
);

insert into student values(1,'Aaron',78);
insert into student values(2,'Bill',76);
insert into student values(3,'Cindy',89);
insert into student values(4,'Damon',90);
insert into student values(5,'Ella',73);
insert into student values(6,'Frado',61);
insert into student values(7,'Gill',99);
insert into student values(8,'Hellen',56);
insert into student values(9,'Ivan',93);
insert into student values(10,'Jay',90);

commit;

首先，我们来看一下UNION的例子：

 view plaincopyprint?SQL> select *  
  2  from student  
  3  where id<4  
  4  union  
  5  select *  
  6  from student  
  7  where id>2 and id<6  
  8  ;  
  
        ID NAME                                SCORE  
---------- ------------------------------ ----------  
         1 Aaron                                  78  
         2 Bill                                   76  
         3 Cindy                                  89  
         4 Damon                                  90  
         5 Ella                                   73  
  
SQL>  
SQL> select *
  2  from student
  3  where id<4
  4  union
  5  select *
  6  from student
  7  where id>2 and id<6
  8  ;

        ID NAME                                SCORE
---------- ------------------------------ ----------
         1 Aaron                                  78
         2 Bill                                   76
         3 Cindy                                  89
         4 Damon                                  90
         5 Ella                                   73

SQL> 

 

如果换成Union All连接两个结果集，则结果如下：

 view plaincopyprint?SQL> select *  
  2  from student  
  3  where id<4  
  4  union all  
  5  select *  
  6  from student  
  7  where id>2 and id<6  
  8  ;  
  
        ID NAME                                SCORE  
---------- ------------------------------ ----------  
         1 Aaron                                  78  
         2 Bill                                   76  
         3 Cindy                                  89  
         3 Cindy                                  89  
         4 Damon                                  90  
         5 Ella                                   73  
  
6 rows selected.  
SQL> select *
  2  from student
  3  where id<4
  4  union all
  5  select *
  6  from student
  7  where id>2 and id<6
  8  ;

        ID NAME                                SCORE
---------- ------------------------------ ----------
         1 Aaron                                  78
         2 Bill                                   76
         3 Cindy                                  89
         3 Cindy                                  89
         4 Damon                                  90
         5 Ella                                   73

6 rows selected. 

 

可以看到，Union和Union All的区别之一在于对重复结果的处理。



接下来，我们交换一个两个SELECT语句的顺序，看看结果是怎样的。

 view plaincopyprint?SQL> select *  
  2  from student  
  3  where id>2 and id<6  
  4  union  
  5  select *  
  6  from student  
  7  where id<4  
  8  ;  
  
        ID NAME                                SCORE  
---------- ------------------------------ ----------  
         1 Aaron                                  78  
         2 Bill                                   76  
         3 Cindy                                  89  
         4 Damon                                  90  
         5 Ella                                   73  
  
SQL> select *  
  2  from student  
  3  where id>2 and id<6  
  4  union all  
  5  select *  
  6  from student  
  7  wher