mysql 指令

1. LEFT  JOIN
   select  欄位(a.col1,b.col2,c.col5)
   
    from 資料表 as a left join   資料表  as b on   欄位a =   欄位b  left join   資料表  as c on   欄位a =   欄位c
   
   where 撈資料條件
   
   
   
   
   
   
   
   
3. $sql = "select a.nsn,b.nc_title,a.start_day ,a.news_title,a.uid,a.enable_group,a.have_read_group,a.prefix_tag,c.sign_unit from ".$xoopsDB->prefix("tad_news")." as a left join ".$xoopsDB->prefix("tad_news_cate")." as b on a.ncsn=b.ncsn left join ".$xoopsDB->prefix("ugm_sign_news")." as c on a.nsn=c.nsn  where b.not_news='0' and a.enable='1' order by a.start_day desc ";
4.