结论：
当MySQL中字段为int类型时，搜索条件where num=‘111‘ 与where num=111都可以使用该字段的索引。
当MySQL中字段为varchar类型时，搜索条件where num=‘111‘ 可以使用索引，where num=111 不可以使用索引
验证过程：
建表语句：
CREATE TABLE `gyl` ( `id` int(11) NOT NULL AUTO_INCREMENT, `str` varchar(255) NOT NULL, `num` int(11) NOT NULL DEFAULT ‘0‘, `obj` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`), KEY `str_x` (`str`), KEY `num_x` (`num`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
向表中使用自复制语句插入数据
insert into gyl (`str`,`num`)values(123123,‘12313‘);
insert into gyl (`str`,`num`) select `str`,`num` from gyl;
更改数据 update gyl set num=id,str=id
结果：
mysql> explain select * from gyl where str=123123 limit 1; +----+-------------+-------+------+---------------+------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+--------+-------------+ | 1 | SIMPLE | gyl | ALL | str_x | NULL | NULL | NULL | 262756 | Using where | +----+-------------+-------+------+---------------+------+---------+------+--------+-------------+ 1 row in set mysql> explain select * from gyl where str=‘123123‘ limit 1; +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+ | 1 | SIMPLE | gyl | ref | str_x | str_x | 257 | const | 131378 | Using where | +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+ 1 row in set mysql> explain select * from gyl where num=‘12313‘ limit 1;; +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+ | 1 | SIMPLE | gyl | ref | num_x | num_x | 4 | const | 131378 | | +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+ 1 row in set 1065 - Query was empty mysql> explain select * from gyl where num=12313 limit 1; +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+ | 1 | SIMPLE | gyl | ref | num_x | num_x | 4 | const | 131378 | | +----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+ 1 row in set
