left join 左连接 以左表为准，但是如果右表也有查询条件时，需要特别注意：
比如这样，只能获取到左右表都符合条件的数据：
SELECT a.id,b.status FROM bw_home_message a
right join bw_home_message_log as b
on a.id=b.home_message_id
WHERE a.home_id = 0 AND a.type = 2 AND b.home_id = ‘2‘
如果只是以左表为准的话需要修改一下：
SELECT a.id,b.status FROM bw_home_message a
left join
（SELECT home_message_id,status FROM bw_home_message_log where home_id=2） as b
on a.id=b.home_message_id
WHERE a.home_id = 0 AND a.type = 2
Thinkphp中：
$where = array(‘a.home_id‘ => 0, ‘a.type‘ => 2);
$logTable = C(‘DB_PREFIX‘) . ‘home_message_log‘;
$sql = "SELECT home_message_id,status FROM {$logTable} where home_id=" . $home_id;
$info = M(‘home_message‘)->alias(‘a‘)->join("left join ($sql) as b on a.id=b.home_message_id")->where($where)->field(‘a.id,b.status‘)->select();
