sql - 从多个表中选择多个列

Question

例子。
我有两张桌子

CAR
id  name
1    bmw
2    fiat

Car_info
car_id  field_name   country
  1     year          2000
  1     country       germany
  2     year          1988
  2     country       italy

如何在一个选择查询中得到这个？

id name  year  country
1   bmw  2000   germany
2   fiat 1988   italy

Answer 1

理想情况下，您应该在 car_info 表中有一列指定该行上显示的信息类型。即类型 1 是年份，类型 2 是国家。

你可以通过加入 car_info 表两次来解决这个问题，在加入时指定一个“AND”条件，如下所示：

SELECT car.ID, car.Name, ci1.country as [YEAR], ci2.country as COUNTRY
FROM car
INNER JOIN car_info AS ci1 ON Car.ID = ci1.car_id AND ci1.field_name = 'year'
INNER JOIN car_info AS ci2 ON Car.ID = ci2.car_id AND ci2.field_name = 'country'

Answer 2

WITH car AS (
  SELECT 1 AS id, 'BMW' AS name FROM dual
  UNION ALL
  SELECT 2, 'Fiat' FROM dual
)
, car_info AS (
  SELECT 1 AS car_id, 'Year' AS field_name, '2000' AS field_val FROM dual
  UNION ALL
  SELECT 1, 'Country', 'Germany' FROM dual
  UNION ALL
  SELECT 2, 'Year', '1988' FROM dual
  UNION ALL
  SELECT 2, 'Country', 'Italy' FROM dual
)
  SELECT c.*, car_year.field_val AS yr, car_country.field_val AS country
    FROM car c
    JOIN car_info car_year ON c.id = car_year.car_id AND car_year.field_name = 'Year'
    JOIN car_info car_country ON c.id = car_country.car_id AND car_country.field_name = 'Country'
;

Answer 3

试试这个：你可以使用PIVOT然后加入Car表

with cte as 
(
 Select car_id,[year],[country]
 from 
 (Select car_id,field_name,countryName
  from car_info
 )pv
 pivot 
 (
  max(countryName)
  FOR field_name IN ([year],[country])
 )pvt
 )
 Select c.name,ct.year,ct.country
 from car c inner join cte ct
 on ct.car_id=c.id

SQL Fiddle中的演示