Question

我正在尝试创建一个程序，其中给出一个战斗名称，产生两个在该战斗中的国家。（如果两个国家/地区都没有正好两个国家/地区生成NULL）

程序的第一部分对我来说并不重要（我认为）

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;

我想这会给我一张表格，其中列出了具体战斗中不同国家的名单。当我尝试执行第二部分时，我的问题就出现了，如果不是两个国家都发现两个国家都生成NULL。

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;
//

如何正确编写第1行和第2行？其他行是否正确？

关系：

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

Answer 1

您应该使用表连接来简化基本查询。您可以使用一个SELECT查询将3个表连接在一起，获得给定战斗的不同国家/地区列表。

由于您正在编写存储过程，因此一种直接的方法是首先在局部变量中选择计数，然后使用条件显示国家名称或空值。

以下是基于源代码的示例：

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;
//

MySQL程序在给定国家的战斗中找到船只。内在关系

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