如何汇总引用另一个表的2列中的数据，并获得过去3个月的每月总数？

Using the data below:

1. How do I get the sum of the cost of each incident based on two columns (crime_incidentid, similar_incidentid) in the listofincidents table?

2. Also how do I get the sums for the past 3 months (January, February and March)?

   create table crimeincidents (
     id int not null,
     name varchar(20),
     primary key (id)
   );
   
   create table listofincidents (
     id int not null,
     incidentdate datetime not null,
     crime_incidentid int not null,
     similar_incidentid int not null,
     cost_to_city decimal(8,2),
     primary key (id),
     FOREIGN KEY (crime_incidentid) REFERENCES crimeincidents(id),
     FOREIGN KEY (similar_incidentid) REFERENCES crimeincidents(id)
   ); 
   
   insert into crimeincidents  (id,name) values 
     (1,'Burglary'),
     (2,'Theft'),
     (3,'Grand theft auto');
   
   insert into listofincidents (id, incidentdate, crime_incidentid,
     similar_incidentid, cost_to_city) 
    values
     (1, "2018-01-10 18:48:00", 1, 2, 900),
     (2, "2018-02-15 14:48:00", 2, 3, 800),
     (3, "2018-02-20 18:10:00", 3, 1, 1500.10), 
     (4, "2018-03-20 18:48:00", 1, 3, 800.23),
     (5, "2018-03-25 18:24:00", 1, 3, 200.00),
     (6, "2018-04-15 10:12:00", 1, 2, 400.00);
   

The query to generate the results without monthly dates is:

select c.id, c.name, sm.similarIncidentCost, cr.crimeIncidentCost 
  from crimeincidents c
  inner join (
    select c.id, sum(s.cost_to_city) similarIncidentCost 
    from crimeincidents c inner join listofincidents s 
                          on s.similar_incidentid = c.id
    group by c.id
  ) sm on sm.id = c.id
  inner join (
     select c.id, sum(cr.cost_to_city) crimeIncidentCost 
       from crimeincidents c inner join listofincidents cr 
                             on cr.crime_incidentid = c.id
       group by c.id
  ) cr on cr.id = c.id;

I want to generate the costs using the past 3 months data. The final result should look like this:

1. January   | 1500.1   |   1900.23
2. February  | 900      |   800
3. March     | 1800.23  |   1500.1

I think this is what you're asking for :

SELECT DATE_FORMAT(li.incidentdate, '%Y-%m') as date,
ci.name,
SUM(
li.cost_to_city
) as totalCost
FROM crimeincidents ci
JOIN listofincidents li ON ci.id = li.crime_incidentid OR ci.id = li.similar_incidentid
GROUP BY date, ci.id
ORDER BY date

And you can go with :

SELECT CONCAT(YEAR(li.incidentdate), ' ', MONTHNAME(li.incidentdate)) as month,
ci.name,
SUM(
li.cost_to_city
) as totalCost
FROM crimeincidents ci
JOIN listofincidents li ON ci.id = li.crime_incidentid OR ci.id = li.similar_incidentid
GROUP BY month, ci.id
ORDER BY month

To match your request better.

Didn't notice at first you wanted "incident" and "similar incident" sums separated. Although I find it weird (since a similar incident can himself have a similar incident) I did the query :

SELECT CONCAT(YEAR(li.incidentdate), ' ', MONTHNAME(li.incidentdate)) as month,
ci.name,
SUM(
IF(ci.id = li.id, li.cost_to_city,0)
) as totalCostIncident,
SUM(
IF(ci.id = li.similar_incidentid, li.cost_to_city,0)
) as totalCostSimilarIncident
FROM crimeincidents ci
JOIN listofincidents li ON ci.id = li.crime_incidentid OR ci.id = li.similar_incidentid
GROUP BY month, ci.id
ORDER BY month