Question

我有不同类别的用户，以及允许用户在多个类别中的联接表。我的连接表名为categories_users，它由user_id和category_id组成。

我想过滤category1和category2中的用户。例如，我想找到对棒球和足球都感兴趣的每个人。

在PostgreSQL中执行此操作的最佳方法是什么？我有以下工作：

select * from users 
  where users.id IN 
    ( Select categories_users.user_id from categories_users 
      JOIN categories ON categories.id = categories_users.category_id 
      where categories.id = 1 OR categories.parent_id = 1) 
  AND users.id IN 
    (Select categories_users.user_id from categories_users 
    JOIN categories ON categories.id = categories_users.category_id 
    where categories.id = 2 OR categories.parent_id = 2)

然而，这感觉很笨，我想知道是否有更好的方法来做到这一点。我尝试了各种联接，但最终总是在categories_users表中搜索category_id为1和2的行，这是不可能的。

编辑，我实际上还需要搜索类别父级，因此我已将上述查询更改为包含parent_id

Answer 1

只需使用同一个表连接两次（使用别名）：

SELECT u.*
    FROM users u
    JOIN categories_users cu1 ON cu1.user_id = u.id
    JOIN categories_users cu2 ON cu2.user_id = u.id
    WHERE cu1.category_id = 1 AND cu2.category_id = 2

Answer 2

select u.*
from
    users u
    inner join (
        select user_id
        from categories_users
        group by user_id
        having
            bool_or(1 in (category_id, parent_id)) and
            bool_or(2 in (category_id, parent_id))
    ) s on s.user_id = u.id

Answer 3

您还可以在分区上使用COUNT（*），以查看用户在搜索类别集中有多少类别。

我创建了以下示例，以了解如何定义和参数化。我创建了一个函数CREATE SCHEMA test; CREATE TABLE test.categories_users ( category_id BIGINT NOT NULL, user_id BIGINT NOT NULL ); INSERT INTO test.categories_users (user_id, category_id) VALUES (33, 103), (34, 104), (35, 105), (37, 105), (35, 106), (37, 106); CREATE OR REPLACE FUNCTION test.find_users_in_categories(BIGINT[]) RETURNS TABLE ( user_id BIGINT ) AS $$ DECLARE categories ALIAS FOR $1; BEGIN RETURN QUERY SELECT t.user_id FROM ( SELECT cu.user_id, cu.category_id, COUNT(*) OVER (PARTITION BY cu.user_id ) AS cnt FROM test.categories_users AS cu WHERE cu.category_id = ANY(categories) ) AS t WHERE t.cnt = array_length(categories, 1) GROUP BY t.user_id; END; $$ LANGUAGE plpgsql; SELECT * FROM test.find_users_in_categories(ARRAY[105, 106]); DROP SCHEMA test CASCADE;，它接受我们需要用户列表的类别数组。因此，该函数将返回所有给定类别中的所有用户。

解决方案 - 获取在所有给定类别中找到的用户

CREATE SCHEMA test;

CREATE TABLE test.categories (
  category_id BIGINT PRIMARY KEY,
  parent_id BIGINT REFERENCES test.categories(category_id)
);

CREATE TABLE test.categories_users (
  category_id BIGINT NOT NULL REFERENCES test.categories(category_id),
  user_id BIGINT NOT NULL
);

INSERT INTO test.categories
  (category_id, parent_id)
  VALUES
  (100, NULL),
  (101, 100),
  (102, 100),
  (103, 101),
  (104, 101),
  (105, 101),
  (106, NULL);


INSERT INTO test.categories_users
  (user_id, category_id)
  VALUES
  (33, 103),
  (34, 104),
  (35, 105),
  (37, 105),
  (35, 106),
  (37, 106);


CREATE OR REPLACE FUNCTION test.find_users_in_categories(BIGINT[])
  RETURNS TABLE (
    user_id BIGINT
  )
AS
$$
DECLARE
  main_categories ALIAS FOR $1;
BEGIN
  RETURN QUERY
  WITH
    -- get all main categories and subcategories
    RECURSIVE cte_categories (category_id, main_category_id) AS
    (
      SELECT cat.category_id, cat.category_id AS main_category_id
        FROM test.categories AS cat
        WHERE cat.category_id = ANY(main_categories)
      UNION ALL
      SELECT cat.category_id, cte.main_category_id
        FROM cte_categories AS cte
        INNER JOIN test.categories AS cat
          ON cte.category_id = cat.parent_id
    ),
    -- filter main categories that are found as children of other categories
    cte_categories_unique AS
    (
      SELECT cte.*
        FROM cte_categories AS cte
        LEFT JOIN
        (
          SELECT category_id
            FROM cte_categories
            WHERE category_id <> main_category_id
            GROUP BY category_id
        ) AS to_exclude
          ON cte.main_category_id = to_exclude.category_id
        WHERE to_exclude.category_id IS NULL
    ),
    -- compute the count of main categories
    cte_main_categories_count AS
    (
      SELECT COUNT(DISTINCT main_category_id) AS cnt
        FROM cte_categories_unique
    )
  SELECT t.user_id
    FROM
      (
        -- get the users which are found in each category/sub-category then group them under the main category
        SELECT
          cu.user_id,
          cte.main_category_id
        FROM test.categories_users AS cu
        INNER JOIN cte_categories_unique AS cte
          ON cu.category_id = cte.category_id
        GROUP BY cu.user_id, cte.main_category_id
      ) AS t
      GROUP BY t.user_id
      -- filter users that do not have a match on all main categories or their sub-categories
      HAVING COUNT(*) = (SELECT cnt FROM cte_main_categories_count);
END;
$$
LANGUAGE plpgsql;


SELECT * FROM test.find_users_in_categories(ARRAY[101, 106]);

DROP SCHEMA test CASCADE;

编辑 - [递归解决方案]

解决方案 - 获取在所有给定类别和子类别中找到的用户

请参阅以下有关使用JOIN +递归CTE实现解决方案的代码。我使用了JOIN而不是COUNT（），因为它看起来更适合这种情况。

# event_source is an observable of messages
# manager.leaders is an observable of leader election events
# manager.followers is an observable of leader relinquish events
event_source\
    .skip_until(manager.leaders)\
    .take_until(manager.followers)\
    .subscribe(observer)

在postgres中，使用连接表进行AND的最佳方法是什么？

3 个答案: