有两个表A和B，均有key和value两个字段，如果B的key在A中也有，就把B的value替换为A中对应的value

update b b set b.value=(select max(a.value) from a a where b.key=a.key)
where exists(select 1 from a c where b.key=c.key)

update a a,(select a.`key`,b.`value` from a inner join b on a.`key`=b.`key`) b 
set a.`value` = b.`value` where a.`key` = b.`key`