1 select * from userinfo ,dapartment where userinfo.part_id = dapartment.id;2      --左连接: 左边全部显示3      select * from userinfo left join dapartment on userinfo.part_id=dapartment.id;4      --右连接:右边的全部显示5      select * from userinfo right join dapartment on userinfo.part_id=dapartment.id;6      --内连接: 将出现null的一行隐藏7      select * from userinfo inner join dapartment on userinfo.part_id = dapartment.id;