我如何解决这个问题？

Question

I tried everything but I can't solve it, someone can help me cause I would like to understand it for getting more experience.

Answer 1

您必须使用GROUP BY命令：

SELECT 
  id_unit, COUNT(*)
FROM 
  order_amount 
GROUP BY 
  id_unit;

结果：

id_unit COUNT(*)
kg  40
m3  30
pal 30

如果您想为每个订单显示您的脚本：

SELECT 
  m.id_order,
  (select count(*) from order_amount m2 where m2.id_order = m.id_order and m2.id_unit = 'kg') as num_kg,
  (select count(*) from order_amount m2 where m2.id_order = m.id_order and m2.id_unit = 'pal') as num_pal,
  (select count(*) from order_amount m2 where m2.id_order = m.id_order and m2.id_unit = 'pal') as num_m3,
  COUNT(*) as total
FROM 
  order_amount m
GROUP BY 
  id_order;

结果：

id_order    num_kg  num_pal num_m3  total
1   5   2   2   8
2   0   2   2   4
3   6   5   5   14
4   5   0   0   10
5   5   3   3   15

详细信息SQL Fiddle