如何在 SQLite 中重用递归 WITH 子句？

Question

我有这个很好的递归 WITH 子句：

WITH RECURSIVE split(seq, word, str) AS (
        SELECT 0, null, replace('name+one+two+three.jpg', '.jpg', '+')
        UNION ALL SELECT
            seq+1,
            substr(str, 0, instr(str, '+')),
            substr(str, instr(str, '+')+1)
        FROM split WHERE str != ''
    ) SELECT word FROM split where seq>1

输出为：

one
two
three

现在，我如何重用这个子句，应用 SELECT name from Images 代替常量字符串 'name+one+two+three.jpg'？

目标是提取可在整组图像名称中找到的所有唯一“+后缀”字符串。例如，这是示例数据：

DROP TABLE IF EXISTS ImagesTemp;
CREATE TEMP TABLE ImagesTemp (name );
INSERT INTO ImagesTemp (name)
VALUES
  ('IMG_0403+newport+malboro+kool.jpg'),
  ('IMG_0404+camel+newport.JPG'),
  ('IMG_0405+dunhill+doral+malboro.png');
SELECT * from ImagesTemp

预期输出为：

word    count
malboro 2
newport 2
kool    1
dunhill 1
doral   1
camel   1

Answer 1

考虑用 table 中的 select 替换 CTE 的锚点，如下所示：

WITH RECURSIVE split(seq, word, str) AS (
    SELECT 0, null, replace(name, '.jpg', '+')
    FROM images
    UNION ALL 
    SELECT
        seq+1,
        substr(str, 0, instr(str, '+')),
        substr(str, instr(str, '+')+1)
    FROM split 
    WHERE str != ''
) 
SELECT word FROM split WHERE seq>1

Answer 2

想通了！这里的技巧是确保文件名以“+”结尾，这样递归子句就不会挂起。我还添加了文件扩展名的计数。

WITH RECURSIVE split(seq, word, str, filename) AS (
    SELECT 0, null, lower(replace(name, '.', '+.')||'+'), name from ImagesTemp
    UNION ALL 
    SELECT
        seq+1,
        substr(str, 0, instr(str, '+')),
        substr(str, instr(str, '+')+1),
        filename
    FROM split 
    WHERE str != ''
) 
SELECT distinct word, count(*) as count, filename as sample
FROM split WHERE word != '' and seq>1
GROUP BY word
ORDER BY count DESC

结果：

word    count   sample
newport 2   IMG_0403+newport+malboro+kool.jpg
malboro 2   IMG_0403+newport+malboro+kool.jpg
.jpg    2   IMG_0404+camel+newport.JPG
kool    1   IMG_0403+newport+malboro+kool.jpg
dunhill 1   IMG_0405+dunhill+doral+malboro.png
doral   1   IMG_0405+dunhill+doral+malboro.png
camel   1   IMG_0404+camel+newport.JPG
.png    1   IMG_0405+dunhill+doral+malboro.png

如何在 SQLite 中重用递归 WITH 子句？

How to reuse a recursive WITH clause in SQLite?

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common-table-expression