mysql中left join设置条件在on与where时的用法区别分析

CREATE TABLE `a` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT 'ID',
 `name` varchar(32) DEFAULT '' COMMENT '名称',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `b` (
 `id` int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT 'ID',
 `a_id` int(11) DEFAULT '0' COMMENT 'a表ID',
 `name` varchar(32) DEFAULT '' COMMENT '名称',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;