mysql从n个最后一行中选择

Question

我有一个索引(自动增量)和整数值的表.该表长达数百万行.

如何最有效地搜索表格的最后n行中是否出现某个数字？

Answer 1

从@chaos给出的答案开始,但稍作修改:

• ORDER BY如果使用,应始终使用LIMIT.RDBMS表没有保证隐式顺序.您通常可以按主键的顺序获取行,但您不能依赖它,也不能移植.

• 如果按降序排序,则无需事先知道表中的行数.

• 您必须为派生表提供相关名称(也称为表别名).

这是我的查询版本:

SELECT `id`
FROM (
    SELECT `id`, `val`
    FROM `big_table`
    ORDER BY `id` DESC
    LIMIT $n
) AS t
WHERE t.`val` = $certain_number;

Answer 2

可能是一个非常晚的答案,但这很好,很简单.

select * from table_name order by id desc limit 5

此查询将返回您在表中插入的最后5个值(最后5行)的集合

Answer 3

最后5行在mysql中检索

此查询工作正常

SELECT * FROM (SELECT * FROM recharge ORDER BY sno DESC LIMIT 5)sub ORDER BY sno ASC

要么

select sno from(select sno from recharge order by sno desc limit 5) as t where t.sno order by t.sno asc

Answer 4

像分页那样充分利用SORT和LIMIT.如果你想要第i个行块,请使用OFFSET.

SELECT val FROM big_table
where val = someval
ORDER BY id DESC
LIMIT n;

响应Nir:排序操作不一定受到惩罚,这取决于查询计划器的功能.由于此用例对分页性能至关重要,因此有一些优化(参见上面的链接).这在postgres中也是如此,"ORDER BY ... LIMIT可以在没有排序的情况下完成" E.7.1.最后一颗子弹

explain extended select id from items where val = 48 order by id desc limit 10;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | items | const | PRIMARY       | PRIMARY | 4       | const |    1 | Using index | 
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+