Question

SELECT SOME_COLUMN_NAME_1
FROM (
      SELECT ROWNUM rnum, SOME_COLUMN_NAME_1
      FROM SOME_TABLE_NAME
      ORDER BY ROWID
)
WHERE rnum between 20001 and 30000

给了我rownum 20001和30000之间的10000行，没有在rownum列上排序

但

SELECT SOME_COLUMN_NAME_1 
FROM ( 
       SELECT ROWNUM rnum,a.SOME_COLUMN_NAME_1 
       FROM (
            SELECT SOME_COLUMN_NAME_1 
            FROM  SOME_TABLE_NAME
       ) a
)
WHERE rnum BETWEEN 20001 AND 30000

在rownum 20001和30000之间给出了10000行，在rownum列

上排序

PLZ解释

Answer 1

线索在于here

Oracle ROWID唯一标识行驻留在磁盘上的位置。 ROWID中的信息为Oracle提供了查找行所需的所有内容，磁盘编号，柱面，块和偏移量。

Rownum是一个伪列。它并不总是与记录有关。它只是按照获取的顺序依次分配的数字。

ROWNUM和ROWID之间的区别在于ROWNUM是临时的，而ROWID是永久性的。另一个区别是ROWID可用于获取行，而ROWNUM仅在单个SQL语句的上下文中有意义，这是一种在获取的结果集中引用行的方法。

所以在你的第一个查询中，当你通过rowid订购然后按rownum过滤时，你会得到一个

SELECT SOME_COLUMN_NAME_1
FROM (
      SELECT ROWNUM rnum, SOME_COLUMN_NAME_1
      FROM SOME_TABLE_NAME
      ORDER BY ROWID
)
WHERE rnum between 20001 and 30000

-- Rows fetched first and assinged a sequence 1,2,...N 
-- they are ordered by rowid (address of the record) make 
-- it a scrambled arrangement

在第二个查询中，

SELECT SOME_COLUMN_NAME_1 
FROM ( 
       SELECT ROWNUM rnum,a.SOME_COLUMN_NAME_1 
       FROM (
            SELECT SOME_COLUMN_NAME_1 
            FROM  SOME_TABLE_NAME
       ) a
)
WHERE rnum BETWEEN 20001 AND 30000;

-- You select a set of rows (Order is preserved)
-- assign a sequence using rownum (Still order is preserved)

oracle查询使用伪列ROWNUM

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