1.null=null不成立，null<>null也不成立
2.
运行汇总当字段为null时，出现bug，及解决思路
join的字段为null，不会出现p1.id=p2.id
mysql> select * from yunxing_huizong;
+------+------+------+
| id | cost | year |
+------+------+------+
| 1 | 1 | 2016 |
| 1 | 2 | 2017 |
| NULL | 2 | 2016 |
| NULL | 3 | 2017 |
+------+------+------+
=============================================
汇总
mysql> select id,sum(cost),year from(select p2.id,p2.cost,p1.year from
(select * from yunxing_huizong)p1 left join
(select * from yunxing_huizong)p2 on p1.id=p2.id and p1.year>=p2.year)p group by id,year;
+------+-----------+------+
| id | sum(cost) | year |
+------+-----------+------+
| NULL | NULL | 2016 |
| NULL | NULL | 2017 |
| 1 | 1 | 2016 |
| 1 | 3 | 2017 |
+------+-----------+------+
===============================================
修改bug思路
将id为null的手动赋值为0或unknown，求出最终结果之后，再将id为0或unknown的改为null，即可
mysql> select case when id is NULL then 0 else id end as id,cost,year from yunxing_huizong;
+----+------+------+
| id | cost | year |
+----+------+------+
| 1 | 1 | 2016 |
| 1 | 2 | 2017 |
| 0 | 2 | 2016 |
| 0 | 3 | 2017 |
+----+------+------+
mysql> select id,sum(cost),year from
(select p2.id,p2.cost,p1.year from
(select case when id is NULL then "unkow" else id end as id,cost,year from yunxing_huizong)p1 left join
(select case when id is NULL then "unkow" else id end as id,cost,year from yunxing_huizong)p2 on p1.id=p2.id and p1.year>=p2.year)p group by id,year;
+-------+-----------+------+
| id | sum(cost) | year |
+-------+-----------+------+
| 1 | 1 | 2016 |
| 1 | 3 | 2017 |
| unkow | 2 | 2016 |
| unkow | 5 | 2017 |
+-------+-----------+------+
得到正确结果
mysql> select case when id="unkow" then null else id end as id,`sum(cost)` sum_cost,year from (select id,sum(cost),year from (select p2.id,p2.cost,p1.year from (select case when id is NULL then "unkow" else id end as id,cost,year from yunxing_huizong)p1 left join (select case when id is NULL then "unkow" else id end as id,cost,year from yunxing_huizong)p2 on p1.id=p2.id and p1.year>=p2.year)p group by id,year)pp;
+------+----------+------+
| id | sum_cost | year |
+------+----------+------+
| 1 | 1 | 2016 |
| 1 | 3 | 2017 |
| NULL | 2 | 2016 |
| NULL | 5 | 2017 |
+------+----------+------+