MySQL 使用 IN/FIND_IN_SET 在子查询中读取多行

Question

我有两个表，位置和位置组

CREATE TABLE locations (
    location_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(63) UNIQUE NOT NULL
);

INSERT INTO locations (name)
  VALUES
('london'),
('bristol'),
('exeter');

CREATE TABLE location_groups (
    location_group_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    location_ids VARCHAR(255) NOT NULL,
    user_ids VARCHAR(255) NOT NULL,
    name VARCHAR(63) NOT NULL,
);

INSERT INTO location_groups (location_ids, user_ids, name)
  VALUES
('1', '1,2,4', 'south east'),
('2,3', '2', 'south west');

我想要做的是返回给定 user_id 存在的所有 location_groups 的所有 location_ids。我正在使用 CSV 将 location_ids 和 user_ids 存储在 location_groups 表中。我知道这不是规范化的，但这就是数据库的方式，它超出了我的控制范围。

我目前的查询是:

SELECT location_id 
  FROM locations 
  WHERE FIND_IN_SET(location_id, 
      (SELECT location_ids 
         FROM location_groups 
         WHERE FIND_IN_SET(2,location_groups.user_ids)) )

现在，如果 user_id = 1 例如(因为只返回 1 个 location_group 行)，这可以正常工作，但是如果我搜索 user_id = 2，我会收到一条错误消息，说子查询返回超过 1 行，这是预期的用户 2位于 2 个 location_groups 中。我明白为什么会抛出错误，我正在尝试解决它。

为了说明在 location_groups.user_ids 中搜索 user_id 1 时，应返回 location_id 1。搜索 user_id 2 时，应返回 location_ids 1,2,3。

我知道这是一个复杂的查询，所以如果有什么不清楚的，请告诉我。任何帮助，将不胜感激!谢谢你。

Answer 1

您可以使用 GROUP_CONCAT 结合 location_id s 在子查询中。

SELECT location_id 
FROM locations 
WHERE FIND_IN_SET(location_id, 
    (SELECT GROUP_CONCAT(location_ids)
     FROM location_groups 
     WHERE FIND_IN_SET(2,location_groups.user_ids)) )

或者，使用编写查询时遇到的问题作为规范化好的原因的示例。哎呀，即使你确实使用了这个查询，它的运行速度也会比在正确规范化的表上的查询慢；你可以用它来说明为什么应该重组表。

作为引用(和其他读者)，这里是规范化模式的样子(包括对基表的一些额外更改)。
location_groups 中的复合字段表可以简单地分成额外的行来实现 1NF ，但这不会在 2NF 中，作为 name列将仅依赖于 location (location, user)的一部分候选键。 (另一种思考方式是 name 是区域的属性，而不是区域/组、位置和用户之间的关系。)

相反，这些列将被拆分为 1NF 的两个附加表:一个连接位置和区域，另一个连接用户和区域。后者可能是用户和位置(而不是区域)之间的关系，但当前模式并非如此(这可能是当前非规范化模式的另一个问题)。区域-位置关系是一对多的(因为每个位置都在一个区域内)。从样本数据中，我们看到区域-用户关系是多对多的。 location_groups表然后变成 region table 。

-- normalized from `location_groups`
CREATE TABLE regions (
    `id` INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    `name` VARCHAR(63) UNIQUE NOT NULL
);

-- slightly altered from original
CREATE TABLE locations (
    `id` INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    `name` VARCHAR(63) UNIQUE NOT NULL
);

-- missing from original sample
CREATE TABLE users (
    `id` INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    `name` VARCHAR(63) UNIQUE NOT NULL
);

-- normalized from `location_groups`
CREATE TABLE location_regions (
    `region` INT UNSIGNED,
    `location` INT UNSIGNED UNIQUE NOT NULL,
    PRIMARY KEY (`region`, `location`),
    FOREIGN KEY (`region`)
        REFERENCES regions (id)
        ON DELETE restrict ON UPDATE cascade,
    FOREIGN KEY (`location`)
        REFERENCES locations (id)
        ON DELETE cascade ON UPDATE cascade
);

-- normalized from `location_groups`
CREATE TABLE user_regions (
    `region` INT UNSIGNED NOT NULL,
    `user` INT UNSIGNED NOT NULL,
    PRIMARY KEY (`region`, `user`),
    FOREIGN KEY (`region`)
        REFERENCES regions (id)
        ON DELETE restrict ON UPDATE cascade,
    FOREIGN KEY (`user`)
        REFERENCES users (id)
        ON DELETE cascade ON UPDATE cascade
);

样本数据:

INSERT INTO regions
  VALUES
('South East'),
('South West'),
('North East'),
('North West');

INSERT INTO locations (`name`)
  VALUES
('London'),
('Bristol'),
('Exeter'),
('Hull');

INSERT INTO users (`name`)
  VALUES
('Alice'),
('Bob'),
('Carol'),
('Dave'),
('Eve');

------ Location-Region relation ------
-- temporary table used to map natural keys to surrogate keys
CREATE TEMPORARY TABLE loc_rgns (
    `location` VARCHAR(63) UNIQUE NOT NULL
    `region` VARCHAR(63) NOT NULL,
);

-- Hull added to demonstrate correctness of desired query
INSERT INTO loc_rgns (region, location)
  VALUES
('South East', 'London'),
('South West', 'Bristol'),
('South West', 'Exeter'),
('North East', 'Hull');

-- map natural keys to surrogate keys for final relationship
INSERT INTO location_regions (`location`, `region`)
  SELECT loc.id, rgn.id
    FROM locations AS loc
      JOIN loc_rgns AS lr ON loc.name = lr.location
      JOIN regions AS rgn ON rgn.name = lr.region;

------ User-Region relation ------
-- temporary table used to map natural keys to surrogate keys
CREATE TEMPORARY TABLE usr_rgns (
    `user` INT UNSIGNED NOT NULL,
    `region` VARCHAR(63) NOT NULL,
    UNIQUE (`user`, `region`)
);

-- user 3 added in order to demonstrate correctness of desired query
INSERT INTO usr_rgns (`user`, `region`)
  VALUES
(1, 'South East'),
(2, 'South East'),
(2, 'South West'),
(3, 'North West'),
(4, 'South East');

-- map natural keys to surrogate keys for final relationship
INSERT INTO user_regions (`user`, `region`)
  SELECT user, rgn.id
    FROM usr_rgns AS ur
      JOIN regions AS rgn ON rgn.name = ur.region;

现在，对规范化架构的所需查询:

SELECT DISTINCT loc.id
FROM locations AS loc
  JOIN location_regions AS lr ON loc.id = lr.location
  JOIN user_regions AS ur ON lr.region = ur.region
;

结果:

+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
+----+