mysql - 计算表中的出现次数，然后用于查找平均值

Question

表格1

categoryid   categoryname   categorydescription
1               a             zzzzzzz
2               b              yyyyyy
3               c               uuuuu

表 2

carid caryear carmodel carprice catid(foreign key to category id )
1        xxxx    aaaa    xxxx    1
2       xxxx     bbbb    xxxx    3
3       xxxx     cccc    xxxx    4
3       xxxx     dddd    xxxx    3
4       xxxxx    eeee    xxxx    1

结果

categoryname                               averageprice                         total cars
a               sum price of same category car / no of same category cars           1
b               sum price of same category car / no of same category cars           2
c               sum price of same category car / no of same category cars           2

Answer 1

你可以写：

 SELECT category.categoryname,
        AVG(car.carprice) AS "averageprice",
        COUNT(car.carid) AS "total cars"
   FROM category
   LEFT
  OUTER
   JOIN car
     ON car.catid = category.categoryid
  GROUP
     BY category.categoryname
;

笔记：

• 你没有提到你的桌子的名字，所以我不得不猜测。
• 这将包括没有任何汽车的类别。如果您只想包含至少有一辆汽车的类别，请删除该LEFT OUTER部分。