请教一句sql语句

Question

在pw_members表里
uid    username
42      lbj
43      xxk
44      fghj
.....在pw_posts表里
author    authorid
lbj         0
xxk         0
fghj        0
.....请问如何用sql语句 把uid的值更新到authorid里呢？
使pw_posts结果为
author    authorid
lbj        42
xxk        43
fghj       44

Answer 1

update pw_posts set authorid=uid
from pw_members where username=author

Answer 2

update pw_posts set authorid=(select uid from pw_members where username=pw_posts.author)

Answer 3

update pw_posts set authorid=uid from pw_posts a,pw_members b
where a.username=b.author

Answer 4

update a 
set authorid=uid 
from pw_posts a,pw_members b
where a.username=b.author