UPDATE语句：将一个表里的字段更新到另一个表的字段里的语句

id subject_id
1 null
2 null
3 null
4 null
5 null
6 null

sb_id st_id
5 1
2 2
5 3
5 4
2 5
2 6

1
UPDATEASETA.subject_id=B.sb_idFROMBWHEREB.student_id=A.id

1
UPDATEA, BSETA.subject_id=B.sb_idWHEREB.student_id=A.id

update table2 b,(select b.area_id as arid,sum(a.user_amount) as bcount
from table1 a,table2 b
where a.user_area=b.area_id
group by arid) c
set b.count=c.bcount
where b.area_id=c.arid;