一个SQL查询问题

Question

有一张票据表,有下面这些字段
id   writeDate(开票日期)      custom(客户号)    type(票据类型)  unit(开票单位)
00000001    2005-11-12                0101001            01            001
00000002    2005-11-22                0101001            01         001
00000003    2005-11-25                0101001            02         001
00000004    2005-12-12                0101001            01         002
00000005    2005-10-12                0101001            04        002
00000006    2005-12-08                0101001            01         003
00000007    2005-10-11                0101001            03
00000008    2005-07-22                0101001            01
00000009    2005-05-12                0101001            01
00000010    2005-05-22                0101001            05
00000011    2005-11-12                0101001            01
00000012    2005-11-22                0101015            01
00000013    2005-11-25                0101001            02
00000014    2005-12-12                0101001            01
00000015    2005-10-12                0101001            04
00000016    2005-12-08                0101025            01
00000017    2005-10-11                0101055            03
00000018    2005-07-22                0101001            01
00000019    2005-05-12                0101001            01
00000020    2005-05-22                0101015            05
我现在要求根据一个时间段选出某个单位每个月开出的票的总数,启号和止号,比如
时间                  起号 止号    总数
2005-11             00000001         00000003     3 
2005-12             00000004         00000005     2 

Answer 1

select  writeDate, min(id), max(id), max(id)-min(id)+1
from t
where ......
group by writeDate

Answer 2

select  writeDate, min(id), max(id), max(id)-min(id)+1
from t
where extract(month from writeDate) As month
group by month

Answer 3

select  unit,to_char(writeDate,'yyyy-mm'), min(id), max(id), count(*)
from t
group by unit,to_char(writeDate,'yyyy-mm')