备注:测试数据库版本为MySQL 8.0

一.需求

如果表中既有库存，又有出货记录，根据表type字段来判断，想想求出截止当前出货时间的当前库存

测试数据:

drop table t;

create table t(id int,type varchar(100),amt decimal(15,2));

insert into t values (1,'库存',100000.00);
insert into t values (2,'出货',2000.00);
insert into t values (3,'出货',8000.00);
insert into t values (4,'出货',10000.00);
insert into t values (5,'出货',15000.00);

二.解决方案

其实和累计和差不多，只是将库存沿用整数，出货用负数来表示即可

-- 普通写法
SELECT t.id,t.type,t.amt 
       ,case when t.type = '库存' then t.amt else -t.amt end as new_amt
       ,( select sum( case when t2.type = '库存' then t2.amt else -t2.amt end ) from t t2 where t2.id <= t.id) lj_amt
from t
order by id

-- 分析函数写法
SELECT t.id,t.type,t.amt 
       ,case when t.type = '库存' then t.amt else -t.amt end as new_amt
       ,sum(case when t.type = '库存' then t.amt else -t.amt end) over (order by id) lj_amt
from t
order by id

测试记录

mysql> SELECT t.id,t.type,t.amt
    ->        ,case when t.type = '库存' then t.amt else -t.amt end as new_amt
    ->        ,( select sum( case when t2.type = '库存' then t2.amt else -t2.amt end ) from t t2 where t2.id <= t.id) lj_amt
    -> from t
    -> order by id;
+------+--------+-----------+-----------+-----------+
| id   | type   | amt       | new_amt   | lj_amt    |
+------+--------+-----------+-----------+-----------+
|    1 | 库存   | 100000.00 | 100000.00 | 100000.00 |
|    2 | 出货   |   2000.00 |  -2000.00 |  98000.00 |
|    3 | 出货   |   8000.00 |  -8000.00 |  90000.00 |
|    4 | 出货   |  10000.00 | -10000.00 |  80000.00 |
|    5 | 出货   |  15000.00 | -15000.00 |  65000.00 |
+------+--------+-----------+-----------+-----------+
5 rows in set (0.00 sec)

mysql> SELECT t.id,t.type,t.amt
    ->        ,case when t.type = '库存' then t.amt else -t.amt end as new_amt
    ->        ,sum(case when t.type = '库存' then t.amt else -t.amt end) over (order by id) lj_amt
    -> from t
    -> order by id;
+------+--------+-----------+-----------+-----------+
| id   | type   | amt       | new_amt   | lj_amt    |
+------+--------+-----------+-----------+-----------+
|    1 | 库存   | 100000.00 | 100000.00 | 100000.00 |
|    2 | 出货   |   2000.00 |  -2000.00 |  98000.00 |
|    3 | 出货   |   8000.00 |  -8000.00 |  90000.00 |
|    4 | 出货   |  10000.00 | -10000.00 |  80000.00 |
|    5 | 出货   |  15000.00 | -15000.00 |  65000.00 |
+------+--------+-----------+-----------+-----------+
5 rows in set (0.00 sec)