如何在MYSQL中执行此查询

Question

您好我有创建SQL的问题。我有表格和这样的数据：

id month year id_type qty
1   10   2012   1      5
2   10   2012   2      4
3   10   2012   3      3 
4   10   2012   4      5
5   11   2012   1      1
6   11   2012   2      2
7   12   2012   1      3
8   12   2012   2      2

我想创建SQL来生成像这样的数据

id_type month year qty month_b4 year_b4 qty_b4
  1       10   2012  5    9       2012    0
  2       10   2012  4    9       2012    0        
  3       10   2012  3    9       2012    0        
  4       10   2012  5    9       2012    0        
  1       11   2012  1    10      2012    5
  2       11   2012  2    10      2012    4
  1       12   2012  1    11      2012    1                
  2       12   2012  2    11      2012    2

Answer 1

此查询将执行此操作：

select 
  sales.id_type as id_type,
  sales.month as month,
  sales.year as year,
  sales.qty as qty,
  prev.month as month_b4,
  prev.year as year_b4,
  prev.qty as qty_b4
from 
  sales,
  sales as prev
where
  sales.id_type = prev.id_type and
  ((sales.month=1 and prev.month=12 and prev.year=sales.year-1) or 
     prev.month = sales.month - 1);

我也可以在小提琴上练习它： http://sqlfiddle.com/#!2/ed3af/14