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  • Oracle——集合运算

    集合运算

    示例 1:查询10和20号部门的员工

    1. select * from emp where deptno=10 or deptno=20;
    2. select * from emp where deptno in (10,20);
    3. 集合运算
        select * from emp where deptno=10
        union
        select * from emp where deptno=20;

    示例2:group by中的rollup函数

      如果是Group by  ROLLUP(A, B, C)的话,首先会对(A、B、C)进行GROUP BY,然后对(A、B)进行GROUP BY,然后是(A)进行GROUP BY,最后对全表进行GROUP BY操作。

          如果是GROUP BY CUBE(A, B, C),则首先会对(A、B、C)进行GROUP BY,然后依次是(A、B),(A、C),(A),(B、C),(B),(C),最后对全表进行GROUP BY操作。

    SELECT
    	deptno,
    	job,
    	sum( sal ) 
    FROM
    	emp 
    GROUP BY
    	rollup ( deptno, job );
    	
    -- 相当于
    
    select deptno,job,sum(sal) from emp group by deptno,job
    union
    select deptno,**to_char(null)**,sum(sal) from emp group by deptno
    union
    select **to_number(null),to_char(null)**,sum(sal) from emp;
    

    注意的问题:

    1、参与运算的各个集合必须列数相同 且类型一致

    2、采用第一个集合作为最后的表头

    3、order by永远在最后

    4、括号

    SQL优化 4、 尽量不要使用集合运算

    集合运算总结:

    -- 将t1的结果和t2的结果联合显示。不排序操作,也不去掉重复的行。
    select * from t1
    union all
    select * from t2;
    
    -- 将t1的结果和t2的结果联合显示。含有排序操作,也去掉重复的行。
    select * from t1
    union
    select * from t2;
    
    -- 将t1的结果和t2的结果的共有部分显示。含有排序操作,也去掉重复的行。
    select * from t1
    INTERSECT
    select * from t2;
    
    -- T1表有,而t2表没有的行,去掉重复的行。
    select * from t1
    MINUS
    select * from t2;
    

    高级分组rollup,cube操作:

    rollup分组

      -- 按部门分组
      select deptno,sum(sal) from emp group by deptno;
    DEPTNO SUM(SAL)
    
    30 9400
    
    20 6775
    
    10 8750
    
      -- 按部门分组,并求总计
      select deptno,sum(sal) from emp  group by rollup(deptno);
    DEPTNO SUM(SAL)
    
    10 8750
    
    20 6775
    
    30 9400
    
           24925
    Rollup分组,一次全表扫描
    
      /*
      Group by Rollup(a,b,c,d)的结果集为,共n+1个集
      ​
      Group by a,b,c,d
      Union all
      Group by a,b,c
      Union all
      Group by a,b
      Union all
      Group by a
      Union all
      Group by null
      */
      ​
      select deptno,job,sum(sal) from emp group by rollup(deptno,job);
      ​
      相当于
      ​
      select deptno,job,sum(sal) from emp group by deptno,job
      union all
      select deptno,null,sum(sal) from emp group by deptno
      union all
      select null,null,sum(sal) from emp;

    Cube分组

      select deptno,job,grouping(deptno),grouping(job),sum(sal) from emp group by cube(deptno,job);
      ​
      结果集为,2**n个结果集
      ​
      select deptno,job,sum(sal) from emp group by deptno,job
      ​
      union all
      ​
      select deptno,null,sum(sal) from emp group by deptno
      ​
      union all
      ​
      select null,job,sum(sal) from emp group by job
      ​
      union all
      ​
      select null,null,sum(sal) from emp;

    树结构的查询start with子句

    层次结构的数据查询

    -- 从ename='SMITH'开始查,一层一层的查出empno=mgr的人的信息
    select empno,ename,mgr from emp start with (ename='SMITH')connect by prior mgr=empno;
    
    EMPNO ENAME       MGR
    
    ---
    
     7369 SMITH            7902
    
     7902 FORD             7566
    
     7566 JONES            7839
    
     7839 KING
    
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  • 原文地址:https://www.cnblogs.com/x54256/p/9001191.html
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