MySQL 同表及多表串联(级联)

多表，允空模式

SELECT
    T0.id AS "v",
    CONCAT(
        IFNULL(CONCAT(T2.title, "/"), ""),
        IFNULL(CONCAT(T1.title, "/"), ""),
        IFNULL(T0.title, "")
    ) AS "n"
FROM
    marketing AS T0
LEFT JOIN `source` AS T1 ON T0.marketing_type = T1.id
LEFT JOIN `category` AS T2 ON T1.sort_id = T2.id

同表转多表

SELECT
    CONCAT(T1.title, "/", T0.title) AS "n",
    T0.id AS "v"
FROM
    tags AS T0,
    tags AS T1
WHERE
    T0.parent_id = T1.id
AND T0.keyword = 'learning_direction'
AND T1.keyword = 'learning_direction'