跨年按周分组
Sql代码  
use employees;  
create table sales(  
    id int auto_increment,  
    date datetime not null,  
    cost int unsigned not null,  
    primary key(id)  
);  
  
insert into sales (date,cost)values('2010-12-31',100);  
insert into sales (date,cost)values('2011-01-01',200);  
insert into sales (date,cost)values('2011-01-02',100);  
insert into sales (date,cost)values('2011-01-06',100);  
insert into sales (date,cost)values('2011-01-10',100);  
  
SELECT * FROM employees.sales;  
  
select week(date),sum(cost)from sales group by week(date);  
  
select floor(datediff(date,'1900-01-01')/7) as a,sum(cost) from sales group by floor(datediff(date,'1900-01-01')/7);  
  
/*最终解决方案*/  
select date_add('1900-01-01',interval floor(datediff(date,'1900-01-01')/7)*7 day) as week_start,  
date_add('1900-01-01',interval floor(datediff(date,'1900-01-01')/7)*7+6 day)as week_end,sum(cost) from sales  
group by floor(datediff(date,'1900-01-01')/7);