zoukankan      html  css  js  c++  java
  • limit offset,n揭秘

    今天利用limit offset,n从一个表中分批导出数据的时候,总比原表少那么几条记录,创建一个小表做测试才发现其中的奥秘:

    mysql> select * from sss1;
    +----+------+------+
    | id | name | c_id |
    +----+------+------+
    | 1 | aaa | 1 |
    | 2 | bbb | 2 |
    | 3 | ccc | 3 |
    | 4 | ddd | 1 |
    | 5 | eee | 4 |
    | 6 | ffff | 6 |
    | 7 | NULL | 0 |
    | 8 | NULL | 0 |
    | 9 | NULL | 0 |
    | 10 | ssss | 0 |
    | 11 | ooo | 0 |
    | 12 | 234 | 0 |
    +----+------+------+
    12 rows in set (0.00 sec)

    mysql
    > select * from sss1 limit 0,3;
    +----+------+------+
    | id | name | c_id |
    +----+------+------+
    | 1 | aaa | 1 |
    | 2 | bbb | 2 |
    | 3 | ccc | 3 |
    +----+------+------+
    3 rows in set (0.00 sec)

    mysql
    > select * from sss1 limit 4,6;
    +----+------+------+
    | id | name | c_id |
    +----+------+------+
    | 5 | eee | 4 |
    | 6 | ffff | 6 |
    | 7 | NULL | 0 |
    | 8 | NULL | 0 |
    | 9 | NULL | 0 |
    | 10 | ssss | 0 |
    +----+------+------+
    6 rows in set (0.00 sec)

    看明白了吗?

    如果有offset的话,就会从offset+1条开始显示,越过第offset条,所以分批后我才会少几条记录,刚好是我的offset点,解决办法:offset 和上一条的n保持一致就行了:

    mysql> select * from sss1;
    +----+------+------+
    | id | name | c_id |
    +----+------+------+
    | 1 | aaa | 1 |
    | 2 | bbb | 2 |
    | 3 | ccc | 3 |
    | 4 | ddd | 1 |
    | 5 | eee | 4 |
    | 6 | ffff | 6 |
    | 7 | NULL | 0 |
    | 8 | NULL | 0 |
    | 9 | NULL | 0 |
    | 10 | ssss | 0 |
    | 11 | ooo | 0 |
    | 12 | 234 | 0 |
    +----+------+------+
    12 rows in set (0.00 sec)

    mysql
    > select * from sss1 limit 0,3;
    +----+------+------+
    | id | name | c_id |
    +----+------+------+
    | 1 | aaa | 1 |
    | 2 | bbb | 2 |
    | 3 | ccc | 3 |
    +----+------+------+
    3 rows in set (0.00 sec)

    mysql
    > select * from sss1 limit 3,6;
    +----+------+------+
    | id | name | c_id |
    +----+------+------+
    | 4 | ddd | 1 |
    | 5 | eee | 4 |
    | 6 | ffff | 6 |
    | 7 | NULL | 0 |
    | 8 | NULL | 0 |
    | 9 | NULL | 0 |
    +----+------+------+
    6 rows in set (0.00 sec)

  • 相关阅读:
    BZOJ 1724: [Usaco2006 Nov]Fence Repair 切割木板 贪心 + 堆 + 反向思考
    BZOJ 1715: [Usaco2006 Dec]Wormholes 虫洞 DFS版SPFA判负环
    qqq
    爬虫的盗亦有道Robots协议
    Requests库
    常用的re模块的正则匹配的表达式
    python -服务器与客户端断电续传程序详细介绍
    模拟ssh远程执行命令,粘包问题,基于socketserver实现并发的socket
    python大佬养成计划----基于flask_sqlalchemy的网页显示数据库信息
    python实战----Todo清单续写
  • 原文地址:https://www.cnblogs.com/sunss/p/2049618.html
Copyright © 2011-2022 走看看