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，and?先過濾后連接

比如說 left join where id = 1，則返回 一條數(shù)據(jù)；

left join on id = 1，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