多表之间实现左连接

CPM_TASK_RES表有三个外键，对应CPM_RES_SORT，CPM_RES_TYPE ，CPM_RES 这三个表的主键，想像两个表左连接那样实现四个表的做连接：

方法如下：

 select a.Task_Code, a.Project_Code, a.Res_Num, a.ResSort_Code, b.ResSort_Name,
      a.ResType_Code, c.ResType_Name, a.Res_Code, d .Res_Name

from CPM_TASK_RES as a left join CPM_RES_SORT as b on a.ResSort_Code=b.ResSort_Code left join CPM_RES_TYPE as c on a.ResType_Code=c.ResType_Code left join CPM_RES as d on a.Res_Code=d.Res_Code