比如，如果B表中有满足条件的数据，才修改A的表：

update A set col1 = xxx where exists(select 1 from B where b.col2 = ‘yyy’);

或者两表关联：

update A set col1 = xxx where exists(select 1 from B where b.col2 = ‘yyy’ and b.col1 = a.col1);