mysql用a表数据更新b表

有两个表
表a
chejia  chudeng
LBERCACB7DX4  '2013-08-27 00:00:00'
LSVFK46R5D2  '2013-10-16 00:00:00'
LSVFK46R5D2 null
LZWACAGA7A  '2010-09-02 00:00:00'
LLV2A2A196 null
LVGBE40K7A  '2011-03-24 00:00:00'
LSGSA52S2CY  '2012-01-09 00:00:00'
LJDLAA296CO  '2012-09-20 00:00:00'
LGBG22E09          null
LGBG22E099  '2009-11-28 00:00:00'
表  b
chejia riqi 
LBERCACB7DX4  '2013-08-27 00:00:00'
LSVFK46R5D2  '2013-10-16 00:00:00'
LSVFK46R5D2  '2013-10-16 00:00:00'
LZWACAGA7A  '2010-09-02 00:00:00'
LLV2A2A196  '2007-04-05 00:00:00'
LVGBE40K7A  '2011-03-24 00:00:00'
LSGSA52S2CY  '2012-01-09 00:00:00'
LJDLAA296CO  '2012-09-20 00:00:00'
LGBG22E09  '2009-11-28 00:00:00'
LGBG22E099  '2009-11-28 00:00:00'
表B中有与表a相同的字段chejia,riqi,
想用B中的日期更新到A中chejia相同的riqi中，但是只更新null的部分，不为null的部分不更新。