A表
name count
name1 0
name2 0
name3 0
B表
id name
1 name1
2 name2
3 name2
update A表，B表中name出现的次数，加到A表.count 上面
------解决方案--------------------
update A set [count]=t.num from A inner join
(
select name,COUNT(1) as num from B group by name
) t on A.name=t.name
------解决方案--------------------
UPDATE A表
SET [count] = b.c
FROM A表,
( SELECT name, count(*) c
FROM B表
GROUP BY name
) B
WHERE A表.name = B.name