Mysql 左右连接

实例代码：

$list = pdo_fetchall('SELECT a.id,a.name,a.time,m.id,m.field FROM ' . tablename('user') . " AS a LEFT JOIN ".tablename('group') . " AS m  ON a.id = m.replyid ");

左连接 以左表为标准 到右表寻找匹配数据

SELECT * FROM 　TABLE1 AS a（取别名） LEFT JOIN TABLE2 AS b ON a = b WHERE 条件... ;

右连接  以右表为标准 到左表寻找匹配数据

SELECT * FROM 　TABLE1 AS a（取别名） RIGHT JOIN TABLE2 AS b ON a = b WHERE 条件... ;

左右连接可以相互转换  

内连接关键字 inner join 写法跟以上同理

$sql = 'select a.id,a.ctype,a.news_title,a.sort,a.add_date,m.id AS mid,m.name_class '.'from ntc_floor'." AS a LEFT JOIN ".' ntb_newsclass'." AS m ON a.ctype = m.id "." order by sort limit $offset,$pagesize";
$r = $db->select($sql);