查询SQL：
SELECT
COUNT(DISTINCT u7.usr_idntf)
FROM
users_wap u7
LEFT JOIN users_wap u6 ON u7.usr_idntf = u6.usr_idntf
WHERE u7.access_date = '2012-10-07'
AND u6.access_date = '2012-10-06'
AND u6.usr_idntf IS NULL
users_wap建表SQL：
CREATE TABLE users_wap (
id bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT '主键',
access_date varchar(12) NOT NULL DEFAULT '1970-01-01' COMMENT '访问时间',
usr_idntf varchar(30) NOT NULL DEFAULT '-1' COMMENT '用户标识',
pv mediumint(8) unsigned NOT NULL DEFAULT '0' COMMENT '每个用户访问次数',
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
问题：查询SQL有何错误？