级联更新第1种情况：
update t1, t2
set t1.x = t2.x, t1.y = t2.y
where t1.id = t2.id
级联更新第2种情况：
update t1,
(select * from t2) as t2
set t1.x = t2.x, t1.y = t2.y
where t1.id = t2.id
级联更新第3种情况：
update (select c from t1 group by c) as temp, t2, t1
set t1.x = t2.x, t1.y = t2.y
where t1.id = t2.id and temp.id = t2.id
级联更新第4种情况：
update t1 left join t2 on t1.id = t2.id
set t1.x = t2.x, t1.y = t2.y
where t1.id = xx
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