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：

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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;

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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：

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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.

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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：

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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

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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。