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  • union 和 union all的区别

    union 和 union all的区别

    相同点和不同点

    相同点:
    union和union all 都是对于多个查询结果的并集进行操作
    不同点:
    1.union 不会输出两个结果并集的重复行
    2.union all 会输出两个结果并集的重复行

    实验表

    字段解释:
    xh:学号
    xh:姓名
    nl:年龄

    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);
    

    查看表

    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.
    

    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> 
    

      

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  • 原文地址:https://www.cnblogs.com/Jansens520/p/11466509.html
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