select * from Aa , Bb where a.id=b.id and a.table_par>='20181001' and b.id_number is null
查询出来的空值，C表格里面有数据，需要往B表格里面插入这些数据，C表和B表ID一样。
求大神解答，学习一下。

从需求上看你是要Update B 表格的 id_number:
update B set B.id_number = C.id_number FROM B b left join A a on a.id=b.id left join C c on c.id = b.id where a.table_par >= '20181001' and b.id_number is null

update B set table_par = (select table_par from C where id = B.id)
where
id_number is null
and
id in (select id from A where table_par>='20181001')