原理分析参见：http://blog.csdn.net/wudongxu/article/details/6966052

这里展示下另外一种情况:子查询不是实体表，而是查询结果集

# 表结构
DROP TABLE IF EXISTS `t1`;
CREATE TABLE `t1` (
  `id` INT NOT NULL AUTO_INCREMENT,
  `c1` INT NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB;

# 数据
INSERT INTO `t1`(`c1`) VALUES ('0'),('1'),('2'),('3'),('4');

# SLEEP函数返回值为0
# join 
SELECT SQL_NO_CACHE t1.* FROM `t1` INNER JOIN (SELECT SLEEP(1) AS val ) AS t2 ON t1.`c1`=t2.val;
# 子查询:被执行了5次(t1表符合条件的行数)
SELECT SQL_NO_CACHE t1.* FROM `t1` WHERE t1.`c1`= (SELECT SLEEP(1) AS val ) ;

性能测试结果