Question

我的数据库中有这些表：

TOTAL TABLE：

total_table_id  | person_id | car_id    | year
----------------|-----------|-----------|------
0               | 1         | 4         | 2015
1               | 1         | 2         | 2017
2               | 2         | 0         | 2017
3               | 3         | 3         | 2017
4               | 3         | 4         | 2015

PERSON TABLE：

person_id   | name  | age
------------|-------|-----
0           | John  | 26
1           | Anna  | 41
2           | Sam   | 33
3           | Tim   | 33

CAR TABLE：

car_id  | model | color
--------|-------|-------
0       | A     | red
1       | B     | blue
2       | B     | white
3       | D     | red
4       | C     | black

在下拉列表中选择年份后我想要的是得到这样的东西：

2017：

color   | age   | cars_count
--------|-------|------------
red     | 33    | 2
white   | 41    | 1

这是我目前的查询：

from a in total
join b in person
on a.person_id equals b.person_id
join c in car
on a.car_id equals c.car_id
select new
{
    color = c.color,
    age = b.age,
    cars_count = ? // <--------This is what I don't know how to get it
}).ToList();

任何提示？

Answer 1

您应该使用group by声明

var answer = (from total in totalTable
             where total.year == 2017
             join car in carTable on total.car_id equals car.car_id
             join person in personTable on total.person_id equals person.person_id 
             group person by car.color into sub
             select new  
             {
                 color = sub.Key,
                 age = sub.Max(x => x.age),
                 //or age = sub.Min(x => x.age),
                 //or age = sub.First().age,
                 count = sub.Count()
             }).ToList();

加入，分组和统计同一个查询

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