原因是内层select语句带有limit子句。
原：
update stu_score_childen A set A.fScore='-1' where A.fID in (select B.fID from stu_score_childen B limit 0,50 )
更改后的
update stu_score_childen A set A.fScore='-1' where A.fID in (select C.fID from (select B.fID from stu_score_childen B limit 0,50 ) C)
绕过带有limit的select就可以了