计算 MySQL 值之间的差异 (PV)

Question

我的学士论文有点卡住了，希望你能帮助我。为了评估光伏系统，我需要计算总能量之间的差异。这些会在 MySQL-table 中自动更新，带有时间戳，但没有 ID 号。我需要自动获取这些能量之间的 delta/difference 作为额外的列，以便在 Grafana 中将其可视化。

******************************************
Timestamp            | SB1_AC_Total       | Needed information (delta)
******************************************
2020-06-24 09:32:45  | 11.326.302         |   23

2020-06-24 09:32:02  | 11.326.279         |   22

2020-06-24 09:31:20  | 11.326.257         |   ...

这个列表持续数周。我真的希望你能帮助我，因为我不知道，这是我第一次使用 MySQL。

Answer 1

您如何处理这取决于您使用各种工具的技能组合。例如，我在 'C' 时老了，但在 SQL 时就变绿了。

现在：假设您已经有一个数据库，并且想要创建一个包含额外字段的新数据库，这就是我的大致处理方式。

创建一个新的空数据库，其中包含您想要的字段。

open both databases 

Read all the old database into a program coded in whatever language you are happy with. 
Typically you will access this by executing an SQL statement like

SELECT * from table1 order by timestamp

In general you can now step through the data base line by line. 
The algorithm you want is this

set a variable 'delta' to zero
set a variable 'flag to zero
while(more lines)
    read 'timestamp'
    read 'SB1_AC_Total'
    write 'timestamp', 'SB1_AC_Total', and 'delta' to the new database using sql INSERT command.
    If 'flag' is not zero, set 'delta' to ('SB1_AC_Total' - 'old_data');  endif
    set 'flag' to 1
    set a variable 'old_data' to 'SB1_AC_Total'
endwhile

这将使用新列复制数据库，该列的值为当前值减去先前值。

我怀疑有一种 SQL 方法使用两个选择生成两个临时表，然后从一个和另一个读取并将结果组合成第三个，但这超出了我的范围。

Answer 2

考虑以下代码并将其用作示例：

create table test (time timestamp, total int);

insert into test values (current_timestamp() - interval 2 day, 100);
insert into test values (current_timestamp() - interval 1 day, 120);
insert into test values (current_timestamp(),                  125);

select i.*, i.total - t_outer.total as diff 
from (select t.*, (select max(time) 
                   from test t1 where t1.time < t.time) as last_timestamp
      from test t
) as i 
left join test t_outer
on i.last_timestamp = t_outer.time;

结果你得到类似的东西：

time    total   last_timestamp  diff
"2020-06-23 10:58:21"   120 "2020-06-22 10:58:19"   20
"2020-06-24 10:58:22"   125 "2020-06-23 10:58:21"   5
"2020-06-22 10:58:19"   100 NULL    NULL

编辑：

如果您想将差异值放入新的 table 列中，您可以这样做：

alter table test add column diff int default null;

create table select_bkp as -- you know the code below already
select i.time, i.total - t_outer.total as diff 
from (select t.*, (select max(time) 
                   from test t1 where t1.time < t.time) as last_timestamp
      from test t
) as i 
left join test t_outer
on i.last_timestamp = t_outer.time;

SET SQL_SAFE_UPDATES = 0; -- so all rows can be updated at once

update test t
join select_bkp b
  on t.time = b.time
set t.diff = b.diff;

SET SQL_SAFE_UPDATES = 1; -- enable secure updates

Answer 3

我能够使用以下代码回答我的问题：

ALTER TABLE TABLENAME  ADD SB1_AC_GES_DIFF INT;
DELIMITER $$
CREATE TRIGGER TABLENAME_Trigger
    BEFORE INSERT
    ON TABLENAME FOR EACH ROW
BEGIN
   DECLARE SB1_AC_GES_old INT;  
  SELECT max(SB1_AC_GES) INTO SB1_AC_GES_old FROM TABLENAME;
   SET NEW.SB1_AC_GES_Diff = New.SB1_AC_GES - SB1_AC_GES_old;
END$$    
DELIMITER ;

计算 MySQL 值之间的差异 (PV)

Calculate difference between values with MySQL (PV)

mysql

delta