一、当用一个表中的数据来更新另外一个表中的数据时（两张表要有关联）：
1. update t1 set t1.c2 = t2.c2
from t2 where t1.c1 = t2.c1
2. update t1 set t1.c2 = t2.c2
from t1 inner join t2 on t1.c1 = t2.c1
二、From子句中指定的表的别名不能作为Set column_name子句中被修改的字段的限定符使用
UPDATE titles
SET t.ytd_sales = t.ytd_sales + s.qty
FROM titles t, sales s
WHERE t.title_id = s.title_id
AND s.ord_date = (SELECT MAX(sales.ord_date) FROM sales)
若要使上例合法，请从列名中删除别名 t 或使用本身的表名。
UPDATE titles
SET ytd_sales = t.ytd_sales + s.qty
FROM titles t, sales s
WHERE t.title_id = s.title_id
AND s.ord_date = (SELECT MAX(sales.ord_date) FROM sales)
更新一列：
update mytab a set name=(select b.name from goal b where b.id=a.id)
where exists (select 1 from goal b where b.id=a.id);
更新多列：
update mytab a
set (name,address)=(select b.name,b.address
from goal b
where b.id=a.id)
where exists (select 1
from goal b
where b.id=a.id )
特别是要注意exists后面的语句：）这会让目标行不至于为NULL
更新update多个关联表的SQL写法：
update customers a
set city_name=(select b.city_name from tmp_cust_city b where b.customer_id=a.customer_id)
where exists (select 1
from tmp_cust_city b
where b.customer_id=a.customer_id
)
update 超过2个值
update customers a
set (city_name，customer_type)=(select b.city_name，b.customer_type
from tmp_cust_city b
where b.customer_id=a.customer_id)
where exists (select 1
from tmp_cust_city b
where b.customer_id=a.customer_id
)
--另一种修改形式
UPDATE SO_Item SET SO_Item.Quantity=Quantity-(SELECT SUM(Quantity) FROM @SaleRule_ItemList AS Rule_Item WHERE Rule_Item.ProductSysno=SO_Item.ProductSysno)
FROM @SO_ItemList AS SO_Item
WHERE SO_Item.ProductSysno IN (SELECT ProductSysno FROM @SaleRule_ItemList)