SQL

Question

我正在构建一个 Pokemon API，并且在通过连接点 table 加入时遇到重复行的问题。它是一个 SQL 服务器数据库。我的 tables 的简化版本如下所示：

重新创建我的数据库：

CREATE DATABASE PokeApi;
USE PokeApi;

CREATE TABLE Pokemon (
    PokemonId INT PRIMARY KEY IDENTITY,
    PokemonName VARCHAR(55),
    NationalDexNumber INT,
    PokemonHeight INT,
    PokemonWeight DECIMAL(10,2),
    Category VARCHAR(50),
    Gender VARCHAR(15),
    GenderDifference BIT,
    TypeOne VARCHAR(15),
    TypeTwo VARCHAR(15),
);

CREATE TABLE Abilities (
    AbilityId INT PRIMARY KEY IDENTITY,
    AbilityName VARCHAR(20)
);

CREATE TABLE PokemonAbilitiesMapped (
    PokemonId INT NOT NULL,
    AbilityId INT,
    CONSTRAINT PK_PokemonAbilitiesMapped PRIMARY KEY
    (
        PokemonId,
        AbilityId
    ),
    FOREIGN KEY(PokemonId) REFERENCES Pokemon(PokemonId),
    FOREIGN KEY(AbilityId) REFERENCES Abilities(AbilityId)
);

INSERT INTO Pokemon
VALUES
  ('Venusaur', 3, 79, 220.5, 'Seed', 'Male / Female', 1, 'Grass', 'Poison'),
  ('Mega Venusaur', 3, 94, 342.8, 'Seed', 'Male / Female', 0, 'Grass', 'Poison'),
  ('Gigantamax Venusaur', 3, 945, NULL, 'Seed', 'Male / Female', 1, 'Grass', 'Poison');

INSERT INTO Abilities
VALUES
  ('Overgrow'),
  ('Thick Fat'),
  ('Test');

INSERT INTO PokemonAbilitiesMapped
VALUES
  (1, 1),
  (2, 2),
  (3, 1),
  (3, 3);

我的查询：

SELECT p.PokemonName, p.NationalDexNumber, p.PokemonHeight, p.PokemonWeight,
p.Category, p.Gender, p.GenderDifference, p.TypeOne, p.TypeTwo,
a.AbilityName
FROM Pokemon p
JOIN PokemonAbilitiesMapped pam
ON p.PokemonId = pam.PokemonId
JOIN Abilities a
ON pam.AbilityId = a.AbilityId

输出这个：

PokemonName	NationalDexNumber	PokemonHeight	PokemonWeight	Category	Gender	GenderDifference	TypeOne	TypeTwo	AbilityName
Venusaur	3	79	220.50	Seed	Male / Female	True	Grass	Poison	Overgrow
Mega Venusaur	3	94	342.80	Seed	Male / Female	False	Grass	Poison	Thick Fat
Gigantamax Venusaur	3	945	null	Seed	Male / Female	True	Grass	Poison	Overgrow
Gigantamax Venusaur	3	945	null	Seed	Male / Female	True	Grass	Poison	Test

如您所见，Gigantamax Venusaur 列出了两次，因为它有两个能力。我在想我可以添加另一个名为“AbilityNameTwo”的列，前提是查询的神奇宝贝具有避免重复行的第二种能力。

在这种情况下避免重复数据的最佳方法是什么？如果我应该在加入时添加另一列，我将如何将其写为查询？

Answer 1

连接数据时出现这样的结果是完全正常的。如果你真的想要，你可以拥有“二能力”，但是如果某些神奇宝贝有三能力，或者五能力呢？你如何订购它们？这是一个示例，说明您如何为两种能力执行此操作并按字母顺序排列：

WITH abilitiesMapped AS
(
    SELECT pam.PokemonId
         , a.AbilityName
         , ROW_NUMBER() OVER (PARTITION BY pam.PokemonId 
                                  ORDER BY a.AbilityName) AS abilityNo
      FROM PokemonAbilitiesMapped pam
      JOIN Abilities a
        ON a.AbilityId = pam.AbilityId
)
SELECT p.PokemonName, p.NationalDexNumber, p.PokemonHeight, p.PokemonWeight,
p.Category, p.Gender, p.GenderDifference, p.TypeOne, p.TypeTwo
   , ability1.abilityName AS ability1
   , ability2.abilityName AS ability2
FROM Pokemon p
LEFT OUTER
JOIN abilitiesMapped ability1
  ON ability1.PokemonId = p.PokemonId
 AND ability1.abilityNo = 1
LEFT OUTER
JOIN abilitiesMapped ability2
  ON ability2.PokemonId = p.PokemonId
 AND ability1.abilityNo = 2

CTE abilitiesMapped 获取您的映射和能力，并为每个口袋妖怪 (PARTITION BY pam.PokemonId) 生成一个 abilityNo，其中数字按能力名称 (ORDER BY ...) 排序。然后这个 CTE 被加入两次 abilityNo 1 和 2。LEFT JOIN 很重要，因为口袋妖怪不一定需要有两个能力，在这种情况下你会在 ability2 中得到一个 NULL（INNER JOIN您只会获得具有两种能力的能力，因为 abilityNo = 2 需要存在才能满足连接条件）。

Answer 2

感谢您 post 的创建、插入语句。很有帮助。

为避免能力名称重复，您可以将能力名称设为逗号分隔列表。我正在使用 STR_AGG 生成逗号分隔列表。从 SQL Server 2017 开始可用。对于早期版本，您必须使用 FOR XML PATH 来生成逗号分隔列表。

SELECT p.PokemonName, p.NationalDexNumber, p.PokemonHeight, p.PokemonWeight,
p.Category, p.Gender, p.GenderDifference, p.TypeOne, p.TypeTwo,
STRING_AGG(a.AbilityName,',') AS Abilities
FROM Pokemon p
JOIN PokemonAbilitiesMapped pam
ON p.PokemonId = pam.PokemonId
JOIN Abilities a
ON pam.AbilityId = a.AbilityId
GROUP BY p.PokemonName, p.NationalDexNumber, p.PokemonHeight, p.PokemonWeight,
p.Category, p.Gender, p.GenderDifference, p.TypeOne, p.TypeTwo

PokemonName	NationalDexNumber	PokemonHeight	PokemonWeight	Category	Gender	GenderDifference	TypeOne	TypeTwo	Abilities
Gigantamax Venusaur	3	945	NULL	Seed	Male / Female	1	Grass	Poison	Overgrow,Test
Mega Venusaur	3	94	342.80	Seed	Male / Female	0	Grass	Poison	Thick Fat
Venusaur	3	79	220.50	Seed	Male / Female	1	Grass	Poison	Overgrow

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