sql 按规则生成编号，字母递增

生成规则 HFR+Code+Date
这是生成的主键编号
HFR-xxx-20150101
HFR-xxx-20150104
HFR-xxx-20150109
HFR-xxx-20150111
HFR-xxx-20150112

当Code和Date重复时就在后面加字母区分
HFR-xxx-20150101
HFR-xxx-20150101A
HFR-xxx-20150101B
HFR-xxx-20150101C
HFR-xxx-20150101D
超过26个字母就这样
HFR-xxx-20150101Z
HFR-xxx-20150101ZA
.....
HFR-xxx-20150101ZZA
....
HFR-xxx-20150101ZZZA
如果后面是数据就好办了，但字母就有点难处理了
注意考虑并发情况