MYSQL版本：5.6.24

先创建一个测试表：

CREATE TABLE `tb` ( `id` bigint(20) NOT NULL, `account` varchar(10) DEFAULT NULL, KEY `idx` (`account`,`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8

再插入数据：

INSERT INTO tb(id,account) VALUES(1,'1001'); INSERT INTO tb(id,account) VALUES(2,'1000'); INSERT INTO tb(id,account) VALUES(3,'1002'); INSERT INTO tb(id,account) VALUES(4,'1003'); INSERT INTO tb(id,account) VALUES(5,'1000'); INSERT INTO tb(id,account) VALUES(6,'1000'); INSERT INTO tb(id,account) VALUES(7,'1005'); INSERT INTO tb(id,account) VALUES(8,'1000'); INSERT INTO tb(id,account) VALUES(9,'1006');

查看SQL执行解释：

EXPLAIN SELECT * FROM `tb` WHERE account = '1000' AND id > 1 AND id < 10 ORDER BY id ASC

得到如下结果

id select_type table type possible_keys key key_len ref rows Extra ------ ----------- ------ ------ ------------- ------ ------- ------ ------ -------------------------- 1 SIMPLE tb range idx idx 41 (NULL) 3 Using where; Using index

但是这个SQL查询的返回记录数却是4行，而explain出来的rows为3，为什么呢？

SELECT * FROM `tb` WHERE account = '1000' AND id > 1 AND id < 10 ORDER BY id ASC