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：
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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：
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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
|
