拆分逗号分隔值并将它们映射到 SQLite 中的原始 ID

我有一张桌子叫articles包含以下格式的数据：

id|categories
--+----------
1|123,13,43
2|1,3,15
3|9,17,44,18,3

出于测试目的，您可以使用以下 SQL 命令创建此表：

CREATE TABLE articles(id INTEGER PRIMARY KEY, categories TEXT);
INSERT INTO articles VALUES(1, '123,13,43'), (2, '1,3,15'), (3, '9,17,44,18,3');

现在我想分割的值categories列，结果如下所示：

id|category
--+--------
1|123
1|13
1|43
2|1
2|3
2|15
3|9
3|17
3|44
3|18
3|3

正如你所看到的，我想将原始表带入第一范式 https://en.wikipedia.org/wiki/First_normal_form.

我已经知道如何拆分one以这种方式排，从this https://stackoverflow.com/a/32051164回答。以下代码示例仅获取第二行（即 id=2 的行）并以所需的方式拆分它们：

WITH split(article_id, word, str, offsep) AS
(
    VALUES
    (
        2,
        '',
        (SELECT categories FROM articles WHERE id=2),
        1
    )
    UNION ALL
    SELECT
        article_id,
        substr(str, 0, CASE WHEN instr(str, ',') THEN instr(str, ',') ELSE length(str)+1 END),
        ltrim(substr(str, instr(str, ',')), ','),
        instr(str, ',')
        FROM split
        WHERE offsep
) SELECT article_id, word FROM split WHERE word!='';

当然，这是非常不灵活的，因为文章ID需要被硬编码。所以，现在我的问题是：我必须在上面的 SQLite 代码中添加或更改什么，才能使其对所有行进行操作并输出所需的结果？

经过一番尝试，我终于自己找到了解决方案。它还负责处理具有'' or NULL作为值categories:

-- create temporary table which buffers the maximum article ID, because SELECT MAX can take a very long time on huge databases
DROP TABLE IF EXISTS max_article_id;
CREATE TEMP TABLE max_article_id(num INTEGER);
INSERT INTO max_article_id VALUES((SELECT MAX(id) FROM articles));

WITH RECURSIVE split(article_id, word, str, offsep) AS
(
    VALUES ( 0, '', '', 0 )                                      -- begin with dummy article 0 (which does not actually exist) to avoid code duplication
    UNION ALL
    SELECT
        CASE WHEN offsep==0 OR str IS NULL
            THEN article_id+1                                    -- go to next article if the current one is finished
            ELSE article_id                                      -- and keep the current one in the opposite case
        END,
        CASE WHEN offsep==0 OR str IS NULL
            THEN ''
            ELSE substr(str, 0, CASE WHEN instr(str, ',') THEN instr(str, ',') ELSE length(str)+1 END)
        END,
        CASE WHEN offsep==0 OR str IS NULL                       -- when str==NULL, then there has been a NULL value for the categories cell of the current article
            THEN (SELECT categories FROM articles WHERE id=article_id+1)
            ELSE ltrim(substr(str, instr(str, ',')), ',')
        END,
        CASE WHEN offsep==0 OR str IS NULL                       -- offsep==0 means that the splitting was finished in the previous iteration
            THEN 1                                               -- offsep==1 means that splitting the categories for a new article will begin in the next iteration
            ELSE instr(str, ',')                                 -- the actual string splitting stuff is explained and taken from here: http://stackoverflow.com/a/32051164
        END
        FROM split
        WHERE article_id<=(SELECT * FROM max_article_id)         -- stop getting new articles when the maximum article ID is reached
) SELECT article_id, word AS category FROM split WHERE word!=''; -- only select article_id and word from the result to use output the desired table layout