MySQL使用一张表的字段更新另一张表的字段

update table1 t1 left join table2 t2 on t1.key=t2.key set
t1.field1=t2.field1,
t1.field2=t2.field2,
t1.field3=t2.field3
where t1.field4 is null and t2.field4 > '2017-04-27';