接上篇文章

上面两个SQL以标准SQL语法来描述可能更容易理解：

SQL1:

  SELECT COUNT(*)
           FROM econtract.cm_contracts_single_v smcon
           LEFT JOIN base.bas_regions_v reg ON reg.region_id = smcon.region
                                           AND reg.enable_flag = 'Y'
                                           AND reg.region_type = 2
           LEFT JOIN base.bas_regions_v off ON off.region_id = smcon.office
                                           AND off.enable_flag = 'Y'
                                           AND off.region_type = 3;

SQL2:

 SELECT COUNT(*)
           FROM econtract.cm_contracts_single_v smcon
           LEFT JOIN base.bas_regions_v reg ON reg.region_id = smcon.region                               
           LEFT JOIN base.bas_regions_v off ON off.region_id = smcon.office                             
         WHERE   reg.enable_flag = 'Y'
           AND off.enable_flag = 'Y'
           AND off.region_type = 3
           AND reg.region_type = 2;

二者的区别显而易见。