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;

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：

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：

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。

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  where id<4
  8  ;

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

6 rows selected.

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 where id<4 8 ; ID NAME SCORE ---------- ------------------------------ ---------- 3 Cindy 89 4 Damon 90 5 Ella 73 1 Aaron 78 2 Bill 76 3 Cindy 89 6 rows selected.

可以看到，对于UNION来说，交换两个SELECT语句的顺序后结果仍然是一样的，这是因为UNION会自动排序。而UNION ALL在交换了SELECT语句的顺序后结果则不相同，因为UNION ALL不会对结果自动进行排序。

那么这个自动排序的规则是什么呢？我们交换一下SELECT后面选择字段的顺序（前面使用SELECT *相当于SELECT ID,NAME,SCORE），看看结果如何：

SQL> select score,id,name
  2  from student
  3  where id<4
  4  union
  5  select score,id,name
  6  from student
  7  where id>2 and id<6
  8  ;

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

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

可是看到，此时是按照字段SCORE来对结果进行排序的（前面SELECT *的时候是按照ID进行排序的）。

那么有人会问，如果我想自行控制排序，能不能使用ORDER BY呢？当然可以。不过在写法上有需要注意的地方：

select score,id,name
from student
where id > 2 and id < 7

union

select score,id,name
from student
where id < 4

union

select score,id,name
from student
where id > 8
order by id desc

select score,id,name from student where id > 2 and id < 7 union select score,id,name from student where id < 4 union select score,id,name from student where id > 8 order by id desc

order by子句必须写在最后一个结果集里，并且其排序规则将改变操作后的排序结果。对于Union、Union All、Intersect、Minus都有效。

其他的集合操作符，如Intersect和Minus的操作和Union基本一致，这里一起总结一下：

Union，对两个结果集进行并集操作，不包括重复行，同时进行默认规则的排序；

Union All，对两个结果集进行并集操作，包括重复行，不进行排序；

Intersect，对两个结果集进行交集操作，不包括重复行，同时进行默认规则的排序；

Minus，对两个结果集进行差操作，不包括重复行，同时进行默认规则的排序。

可以在最后一个结果集中指定Order by子句改变排序方式。