union 和 union all的區別

union 和 union all的區別

相同點和不同點

相同點：
union和union all 都是對於多個查詢結果的並集進行操作
不同點：
1.union 不會輸出兩個結果並集的重複行
2.union all 會輸出兩個結果並集的重複行

實驗表

字段解釋：
xh：學號
xh：姓名
nl：年齡

create table student(xh number,xm varchar2(4),nl int);
insert into student values(1,'A',21);
insert into student values(2,'B',21);
insert into student values(3,'A',21);
insert into student values(4,'A',21);
insert into student values(5,'A',21);
insert into student values(6,'C',21);
insert into student values(7,'B',21);

查看錶

SQL> select * from student;

    XH XM           NL
---------- ------------ ----------
     1 A            21
     2 B            21
     3 A            21
     4 A            21
     5 A            21
     6 C            21
     7 B            21

7 rows selected.

SQL> 

例子

union

SQL> select * from student
  2  union
  3  select * from student where xm='A';

    XH XM           NL
---------- ------------ ----------
     1 A            21
     2 B            21
     3 A            21
     4 A            21
     5 A            21
     6 C            21
     7 B            21

7 rows selected.

SQL> 

union all

SQL> select * from student
  2  union all
  3  select * from student where xm='A';

    XH XM           NL
---------- ------------ ----------
     1 A            21
     2 B            21
     3 A            21
     4 A            21
     5 A            21
     6 C            21
     7 B            21
     1 A            21
     3 A            21
     4 A            21
     5 A            21

11 rows selected.

SQL>