UPDATE a SET [数值]=d.[平均数值]  from (select c.[主键],c.[平均数值] from (select a.[主键], avg(b.数值)[平均数值] from B left join A on a.[主键]=b.A表主键 group  by  a.[主键])c where a.[主键]=c.[主键])d

------解决方案--------------------
SQL code


DECLARE @A TABLE(id INT ,VALUE INT)
INSERT INTO @A
SELECT 1,NULL
DECLARE @B TABLE(id INT,A_id INT,method VARCHAR(10),VALUE INT )
INSERT INTO @B
SELECT 999, 1, 'AAA', 122 UNION ALL
SELECT 1000,1, 'BBB', 124
--1,
SELECT [@A].id,AVG([@B].VALUE) AS avg_value 
FROM @A LEFT OUTER JOIN @B ON [@A].id = [@B].A_id
GROUP BY [@A].id
/*
id          avg_value
----------- -----------
1           123
*/
--2
UPDATE @A SET VALUE=t.avg_value
FROM 
(
    SELECT [@A].id,AVG([@B].VALUE) AS avg_value 
    FROM @A LEFT OUTER JOIN @B ON [@A].id = [@B].A_id
    GROUP BY [@A].id
) AS t WHERE [@A].id=t.id
SELECT * FROM  @A
/*
id          VALUE
----------- -----------
1           123
*/

------解决方案--------------------
SQL code

UPDATE a SET [数值]= case when d.[平均数值] is not null then d.[平均数值]  else a.[数值] end  from (select c.[主键],c.[平均数值] from (select *, avg(b.数值)[平均数值] from B left join A on a.主键=b.A表主键 group  by  a.主键)c where a.主键=c.主键)d