union all和union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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01.drop table student;

02.

03.create table student

04.(

05.id int primary key,

06.name nvarchar2(50) not null,

07.score number not null

08.);

09.

10.insert into student values(1,'Aaron',78);

11.insert into student values(2,'Bill',76);

12.insert into student values(3,'Cindy',89);

13.insert into student values(4,'Damon',90);

14.insert into student values(5,'Ella',73);

15.insert into student values(6,'Frado',61);

16.insert into student values(7,'Gill',99);

17.insert into student values(8,'Hellen',56);

18.insert into student values(9,'Ivan',93);

19.insert into student values(10,'Jay',90);

20.

21.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：

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01.SQL> select *

02. 2 from student

03. 3 where id<4

04. 4 union

05. 5 select *

06. 6 from student

07. 7 where id>2 and id<6

08. 8 ;

09.

10. ID NAME SCORE

11.---------- ------------------------------ ----------

12. 1 Aaron 78

13. 2 Bill 76

14. 3 Cindy 89

15. 4 Damon 90

16. 5 Ella 73

17.

18.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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01.SQL> select *

02. 2 from student

03. 3 where id<4

04. 4 union all

05. 5 select *

06. 6 from student

07. 7 where id>2 and id<6

08. 8 ;

09.

10. ID NAME SCORE

11.---------- ------------------------------ ----------

12. 1 Aaron 78

13. 2 Bill 76

14. 3 Cindy 89

15. 3 Cindy 89

16. 4 Damon 90

17. 5 Ella 73

18.

19.6 rows selected.

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；