http://www.jellythink.com/archives/924
FROM ---> JION----->ON------->WHERE------->GROUP BY ------->HAVING------->
SELECT------->DISTINCT---->ORDER BY----->LIMIT
where?是連接了再過濾厅须，and?先過濾后連接
比如說 left join where id = 1食棕，則返回 一條數(shù)據(jù)；
left join on id = 1簿晓，則返回所有主表數(shù)據(jù) 只不過 id 非 1的數(shù)據(jù)標(biāo)注為 null
inner join 就沒有這種問題
