update a set siteid = '4' where ContentID in (
SELECT a.ContentID FROM a left join b on a.ContentID = b.ContentID where  b.nodeid=13)

提示：
#1093 - You can't specify target table 'a' for update in FROM clause  

问了下gump，原来要这么写：

update a, b set a.siteid = '4' where a.ContentID = b.ContentID and b.nodeid=13