set @tmp="";
update table set  field_a=(@tmp:=field_a), field_a=field_b,field_b=@tmp where ……

------解决方案--------------------
建议用sqlyog，直接将多条语句全部执行。http://download.csdn.net/detail/dmtnewtons/4230136
SQL code

update A set A.a="new_data_a",A.b="new_data_b" where A.a="old_data_a" and A.b="old_data_b";
……

------解决方案--------------------
把字段名换了

------解决方案--------------------
棒!

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SQL code

set @tmp="";
update table set  field_a=(@tmp:=field_a), field_a=field_b,field_b=@tmp where ……