#(1)连接和自连接
select g.Name,t.Name,u.Name
from Goods g
inner join Type t
on g,GoodsId=t.GoodsId
inner join Type u
on t.UpId=u.TypeId
where g.Name like "新品%"
and g,Stock>0;
#(2) select字段中有运算符应该不对
select Name,(Stock-cnt) "yuliang"
from Goods g
inner join Type t
on g.GoodsId=t.GoodsId
inner join(
    select GoodsId,count(OrderId) "cnt"
    from Order o
    inner join Goods g
    on o.GoodsId=g.GoodsId
    group by GoodsId
) a
on g.GoodsId=a.GoodsId
where t.Name="化妆品";
#(3) 求出销量第三的值，返回比其大的货物
select g.Name,a.cnt
from Goods g
inner join(
  select GoodsId,count(OrderId)"cnt"
  from Order o
  inner join Goods g
  group by g.GoodsId
  having count(OrderId)>(
    select count(OrderId)
    from Order o
    inner join Goods g
    on o.GoodsId=g.GoodsId
    group by o.GoodsId
    order by count(OrderId)
    limit 2, 1
  ) 
) a
on g.GoodsId=a.GoodsId;

两个重复规则 . +
规则 . 
“123456.@123.com”,
点(.)可以省略，
等同于"123456@123.com“
规则 +
“123456+a@123.com”+
及其后面的字符可以省略，
等同于"123456@123.com”

public quchong(String[] emails){
  Set<String> set=new HashSet<>();
  for(String s:email){
    set.add(change(s));
  }
    for(String s:set){
    System.out.println(s);
  }
}
public String change(String s){
  int flagAt=0;
  int flagAdd=0;
  for(int i=0;i<s.length;i++){
    if(flagAt==1){
      break;
    }
    if(s.chatAt(i)=='.'&&flagAt==0){
      s.chatAt(i)='';
      continue;
    }
    if(s.chatAt(i)=='+'&&flagAt==0){
      s.chatAt(i)='';
      flagAdd=1;
      continue;
    }
    if(flagAdd==1){
        s.chatAt(i)='';
    }
    if(s.chatAt(i)=='@'&&flagAt==0){  
      flagAt=1;
      break;
    }
  }
}