战斗，船舶SQL查询

这是一个关于战舰和他们战斗的战斗模式：战斗，船舶SQL查询

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

典型船舶元组是：

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

这意味着，战舰新泽西是1943年推出;它属于美国，携带9口大小16英寸（内径或内径）的枪支，并加权（以航海术语计算）46,000吨。一个典型的Battles元组是：

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

也就是说，H.M.S.胡德在北大西洋的战斗中沉没。其他可能的结果是'好的'和'损坏的'

问题：列出所有在战斗中相互作战的国家对。每对只有一次名单，并与至上按字母顺序排列第一

回答全国一一列举：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

但它说，不明确的列名。我如何解决它？在此先感谢

来源

2013-10-03 clever_bassi

好，'name = ship'就是问题。名字可能来自a或b，并且从b1或b2出货 –

在您的WHERE子句中，'name = ship'需要合格。另外，在同一场战斗中，你如何区分战场上的敌对国家和盟国？ – RBarryYoung

SQL不知道你正在寻找哪个名字和船。尝试：'SELECT a.country，b.country from ships a，ships b，battles b1，battles b2 WHERE a.name = b1.ship and b1 .battleName = b2.battleName和a.country> b.country' –

那么，name = ship是问题。 name可能是从a或b，并ship从b1或b2

你可以做这样的事情：

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

来源

2013-10-03 22:40:17

你能尝试嵌套查询得到的赢家和输家的表，然后将它们连接战斗名称？

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

来源

2013-10-03 22:43:01 JustinHui

战斗，船舶SQL查询

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