本文解析了在Oracle数据库中使用ROWNUM进行数据行筛选的工作原理。当ROWNUM设置为1时，仅首条记录通过测试并被输出；若设置为2，则首条记录因不满足条件而被排除，后续记录同样无法满足ROWNUM等于2的要求。文章揭示了ROWNUM在实际应用中的行为特征。
SELECT * FROM T WHERE ROWNUM=1 可以查询出来数据，
而SELECT * FROM T WHERE ROWNUM=2不可以查询出来数据。
in the case of where rownum = 1, the first row passes the test, is output and rownum goes
to 2. No other row ever satisfies the predicate and rownum stays at 2 for the rest of
the query.
in the case of where rownum = 2, the first row is rownum 1, it fails. The next row is
ALSO rownum = 1 and likewise fails. And so on. There can be NO row 2 if there is not a
row 1.
to 2. No other row ever satisfies the predicate and rownum stays at 2 for the rest of
the query.
in the case of where rownum = 2, the first row is rownum 1, it fails. The next row is
ALSO rownum = 1 and likewise fails. And so on. There can be NO row 2 if there is not a
row 1.
扫一扫
1341
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
