在Oracle中有和没有KEEP的PARTITION BY

Question

我遇到了两个似乎有相同结果的查询:在分区上应用聚合函数.

我想知道这两个查询之间是否有任何区别:

SELECT empno,
   deptno,
   sal,
   MIN(sal) OVER (PARTITION BY deptno) "Lowest",
   MAX(sal) OVER (PARTITION BY deptno) "Highest"
FROM empl

SELECT empno,
   deptno,
   sal,
   MIN(sal) KEEP (DENSE_RANK FIRST ORDER BY sal) OVER (PARTITION BY deptno) "Lowest",
   MAX(sal) KEEP (DENSE_RANK LAST ORDER BY sal) OVER (PARTITION BY deptno) "Highest"
FROM empl

第一个版本更合乎逻辑,但第二个版本可能是某种特殊情况,也许是一些性能优化.

Answer 1

MIN(sal) KEEP (DENSE_RANK FIRST ORDER BY sal) OVER (PARTITION BY deptno)

该声明可以(大致)从右到左的顺序来考虑:

• OVER (PARTITION BY deptno)意味着将行分成不同的组deptno; 然后
• ORDER BY sal表示对于每个分区,按行排序sal(隐式使用ASC结束顺序); 然后
• KEEP (DENSE_RANK FIRST表示为每个分区的有序行提供(连续)排名(对于排序列具有相同值的行将被赋予相同的排名)并丢弃所有未排在第一位的行; 最后
• MIN(sal) 对于每个分区的剩余行,返回最低工资.

在这种情况下,MIN并且DENSE_RANK FIRST都在sal列上操作,因此将执行相同的操作并且KEEP (DENSE_RANK FIRST ORDER BY sal)是多余的.

但是,如果您使用不同的列作为最小值,那么您可以看到效果:

SQL小提琴

Oracle 11g R2架构设置:

CREATE TABLE test (name, sal, deptno) AS
SELECT 'a', 1, 1 FROM DUAL
UNION ALL SELECT 'b', 1, 1 FROM DUAL
UNION ALL SELECT 'c', 1, 1 FROM DUAL
UNION ALL SELECT 'd', 2, 1 FROM DUAL
UNION ALL SELECT 'e', 3, 1 FROM DUAL
UNION ALL SELECT 'f', 3, 1 FROM DUAL
UNION ALL SELECT 'g', 4, 2 FROM DUAL
UNION ALL SELECT 'h', 4, 2 FROM DUAL
UNION ALL SELECT 'i', 5, 2 FROM DUAL
UNION ALL SELECT 'j', 5, 2 FROM DUAL;

查询1:

SELECT DISTINCT
  MIN(sal) KEEP (DENSE_RANK FIRST ORDER BY sal) OVER (PARTITION BY deptno) AS min_sal_first_sal,
  MAX(sal) KEEP (DENSE_RANK FIRST ORDER BY sal) OVER (PARTITION BY deptno) AS max_sal_first_sal,
  MIN(name) KEEP (DENSE_RANK FIRST ORDER BY sal) OVER (PARTITION BY deptno) AS min_name_first_sal,
  MAX(name) KEEP (DENSE_RANK FIRST ORDER BY sal) OVER (PARTITION BY deptno) AS max_name_first_sal,
  MIN(name) KEEP (DENSE_RANK LAST ORDER BY sal) OVER (PARTITION BY deptno) AS min_name_last_sal,
  MAX(name) KEEP (DENSE_RANK LAST ORDER BY sal) OVER (PARTITION BY deptno) AS max_name_last_sal,
  deptno
FROM test

结果:

| MIN_SAL_FIRST_SAL | MAX_SAL_FIRST_SAL | MIN_NAME_FIRST_SAL | MAX_NAME_FIRST_SAL | MIN_NAME_LAST_SAL | MAX_NAME_LAST_SAL | DEPTNO |
|-------------------|-------------------|--------------------|--------------------|-------------------|-------------------|--------|
|                 1 |                 1 |                  a |                  c |                 e |                 f |      1 |
|                 4 |                 4 |                  g |                  h |                 i |                 j |      2 |

Answer 2

在您的示例中，没有什么区别，因为您的汇总位于您正在排序的同一列上。“ KEEP”的真正点/功效是当您在不同的列上进行汇总和排序时。例如（从其他答案中借用“测试”表）...

SELECT deptno,  min(name) keep ( dense_rank first order by sal desc, name  ) ,
max(sal)
FROM test
group by deptno

;

该查询获取每个部门中薪水最高的人员的姓名。考虑不带“ KEEP”子句的替代项：

SELECT deptno, name, sal
FROM test t
WHERE not exists ( SELECT 'person with higher salary in same department'
                                            FROM test t2  
                                            WHERE t2.deptno = t.deptno
                                            and ((  t2.sal > t.sal )
                                            OR ( t2.sal = t.sal AND t2.name < t.name ) ) )

KEEP子句更简单，更高效（在这个简单的示例中，只有3个一致的获取与34个一致的获取）。