原表：
Ax_2014091
Ali_20140917
20140916
20140918
mm_20140918
MM_20140918
MM_20140913
想得到这样的表
Ax_2014091
Ali_20140917
hk-20140916
hk-20140918
mm_20140918
MM_20140918
MM_20140913
就是在字段没包含“_”的前面添加“hk-”
------解决思路----------------------
update tbl set col = 'hk-' + col where col not like '%[_]%'