摘要：
table a(id, type):id type----------------------------------1 12 13 2table b(id, class):id class---------------------------------1 12 2sql语句1：select a.*, b.* from a left join b on a.id = b.id and a.type = 1;sql语句2：select a.*, b.* from a left join b on a.id = b.id where a.type = 1;sql语句3：select a.*, b 阅读全文
