Question

这是关于战舰及其战斗的模式：

 Ships(name, yearLaunched, country, numGuns, gunSize, displacement)
 Battles(ship, battleName, result)

典型的Ships元组将是：

 ('New Jersey', 1943, 'USA', 9, 16, 46000)

这意味着战舰新泽西州于1943年发射;它属于美国，装有9门16英寸（枪管或枪管内径）的枪，加重（以航空方式移位）46,000吨。战斗的典型元组是：

 ('Hood', 'North Atlantic', 'sunk')

即H.M.S.胡德在北大西洋的战斗中沉没。其他可能的结果是“确定”和“损坏”

问题：列出在战斗中互相争斗的所有国家对。仅列出每对一次，并将其列为首先按字母顺序排在第一位的国家

答案：我写了这个：

SELECT 
    a.country, b.country 
FROM 
    ships a, ships b, battles b1, battles b2 
WHERE 
    name = ship 
    and b1.battleName = b2.battleName 
    and a.country > b.country

但是它的名字含糊不清。我该如何解决？提前致谢

Answer 1

嗯，name = ship就是问题所在。 name可以来自a或b，ship来自b1或b2

你可以这样做：

select distinct s1.country, s2.country
from ships s1
inner join Battles b1 on b.ship = s1.name
inner join Battles b2 on b2.ship <> s1.name and b2.battleName = b1.battleName
inner join ships s2 on s2.name = b2.ship and s2.country < s1.country

Answer 2

您是否可以尝试嵌套查询获取赢家和输家的表格，然后加入他们的战斗名称？

SELECT
WINNER.country as Country1
,LOSER.country as Country2

FROM
(
  SELECT DISTINCT country, battleName
  FROM Battles 
  INNER JOIN Ships ON Battles.ship = Ships.name
  WHERE Battles.Result = 1
) AS WINNER

INNER JOIN 
(
  SELECT DISTINCT country, battleName
  FROM Battles 
  INNER JOIN Ships ON Battles.ship = Ships.name
  WHERE Battles.Result = 0
) AS LOSER

ON WINNER.battlename = LOSER.battlename
ORDER BY WINNER.country

Answer 3

试试这个，我认为它很简洁：

SELECT DISTINCT s1.country, s2.country
FROM ships s1, ships s2, battles b1, battles b2 -- i just list all tables here but you can use the left join etc.
WHERE s1.name=b1.ship AND s2.name=b2.ship AND b1.battleName=b2.battleName -- for the tables to join each other
AND s1.country < s2.country --key condition
ORDER BY s1.country ASC

战斗，船舶SQL查询

3 个答案: