MySQL程序查找给定国家的战舰。内部关系

我正试图创建一个程序，在这个程序中，给定一个战斗名称，生产出两个在那场战斗中的国家。（如果不存在两个国家，则为这两个国家生成NULL）

程序的第一部分对我来说很重要（我想）

我想这会给我一张表格，列出具体战斗中的不同国家。当我尝试做第二部分时，我的问题就出现了，如果不是两个国家都被发现，那么这两个国家的生产都是空的

CREATE PROCEDURE findCountriesInBattle( in inBattle VARCHAR(50) )
BEGIN
    SELECT ship FROM outcomes WHERE battle = inBattle AS t1;
    SELECT DISTINCT class FROM ships WHERE name IN t1 AS t2;
    SELECT DISTINCT country FROM classes WHERE class IN t2 AS t3;
IF(SELECT COUNT(*) FROM t3 < 2 OR SELECT COUNT(*) FROM t3 > 2)
    [show NULL, NULL]; << Line 1
ELSE
    SELECT * FROM t3; << Line 2
END IF;
END;
// 

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您应该使用表联接来简化基本查询。通过一个SELECT查询将3个表连接在一起，您可以获得给定战斗的不同国家的列表

由于您正在编写存储过程，一种简单的方法是首先将计数选择到局部变量中，然后根据需要使用条件来显示国家名称或空值

下面是一个基于源代码的示例：

CREATE PROCEDURE findCountriesInBattle( in inBattle VARCHAR(50) )
BEGIN
   DECLARE v_country_count INT;

  -- count the countries involved in the battle
  SELECT COUNT(DISTINCT classes.country)
  INTO V_COUNTRY_COUNT
  FROM outcomes
  INNER JOIN ships ON ships.name = outcomes.ship
  INNER JOIN classes ON classes.class = ships.class
  WHERE outcomes.battle = inBattle;

  IF (v_country_count = 2)
  THEN
    -- if it was exactly 2, output the names
    SELECT DISTINCT classes.country
    FROM outcomes
    INNER JOIN ships ON ships.name = outcomes.ship
    INNER JOIN classes ON classes.class = ships.class
    WHERE outcomes.battle = inBattle; 
  ELSE
    -- otherwise, output NULL,NULL
    SELECT null
    UNION ALL
    SELECT null;  
  END IF;
END;
// 

---

我最讨厌尝试学习这一点的是，我可以完全理解这一点，我只是不能像编写C++/Java那样轻松地输入/流动它。我猜它缺乏语法知识。。。

classes(class, type, country, numGuns, bore, displacement)
ships( name, class, launched)
battles(name, date)
outcomes(ship, battle, result)

CREATE PROCEDURE findCountriesInBattle( in inBattle VARCHAR(50) )
BEGIN
   DECLARE v_country_count INT;

  -- count the countries involved in the battle
  SELECT COUNT(DISTINCT classes.country)
  INTO V_COUNTRY_COUNT
  FROM outcomes
  INNER JOIN ships ON ships.name = outcomes.ship
  INNER JOIN classes ON classes.class = ships.class
  WHERE outcomes.battle = inBattle;

  IF (v_country_count = 2)
  THEN
    -- if it was exactly 2, output the names
    SELECT DISTINCT classes.country
    FROM outcomes
    INNER JOIN ships ON ships.name = outcomes.ship
    INNER JOIN classes ON classes.class = ships.class
    WHERE outcomes.battle = inBattle; 
  ELSE
    -- otherwise, output NULL,NULL
    SELECT null
    UNION ALL
    SELECT null;  
  END IF;
END;
//