我想创建一个递归遍历表并为每一行构建一个字符串的过程。除了位于层次结构顶部的少数行之外，每行都与另一行相关。
这是我有的：
CREATE TABLE item (
id INT NOT NULL AUTO_INCREMENT,
name VARCHAR(30) NOT NULL,
category INT NULL,
PRIMARY KEY (id),
FOREIGN KEY (category)
REFERENCES item(id)
);
CREATE PROCEDURE get_item()
SELECT *
FROM item;
-- main categories --
INSERT INTO item (name, category) VALUES
('groceries', NULL),
('retail', NULL),
('service', NULL),
('grains', 1),
('produce', 1),
('meats', 1),
('dairy', 1),
('snacks', 1),
('beverages', 1),
('car', 2),
('home', 2),
('clothing', 2),
('electronics', 2),
('chips', 8),
('dip', 8),
('snack bars', 8),
('haircut', 3);
当调用get_item()时，应该给出这个表：
| id | name | category |
|----|--------------|-----------|
| 1 | groceries | NULL |
| 2 | retail | NULL |
| 3 | service | NULL |
| 4 | grains | 1 |
| 5 | produce | 1 |
| 6 | meats | 1 |
| 7 | dairy | 1 |
| 8 | snacks | 1 |
| 9 | beverages | 1 |
| 10 | car | 2 |
| 11 | home | 2 |
| 12 | clothing | 2 |
| 13 | electronics | 2 |
| 14 | chips | 8 |
| 15 | dip | 8 |
| 16 | snack bars | 8 |
| 17 | haircut | 3 |
我希望它看起来像这样：
| id | name | category | path |
|----|--------------|-----------|-----------------------------------|
| 1 | groceries | NULL | groceries |
| 2 | retail | NULL | retail |
| 3 | service | NULL | service |
| 4 | grains | 1 | groceries > grains |
| 5 | produce | 1 | groceries > produce |
| 6 | meats | 1 | groceries > meats |
| 7 | dairy | 1 | groceries > dairy |
| 8 | snacks | 1 | groceries > snacks |
| 9 | beverages | 1 | groceries > beverages |
| 10 | car | 2 | retail > car |
| 11 | home | 2 | retail > home |
| 12 | clothing | 2 | retail > clothing |
| 13 | electronics | 2 | retail > electronics |
| 14 | chips | 8 | groceries > snacks > chips |
| 15 | dip | 8 | groceries > snacks > dip |
| 16 | snack bars | 8 | groceries > snacks > snack bars |
| 17 | haircut | 3 | service > haircut |
我不知道怎么会这样做。
为此使用存储过程是PITA：
DROP PROCEDURE get_item;
DELIMITER //
CREATE PROCEDURE get_item(IN p_category INT, IN p_names TEXT)
BEGIN
DECLARE done INT DEFAULT FALSE;
DECLARE v_id, v_category INT;
DECLARE v_name VARCHAR(30);
DECLARE v_path TEXT;
DECLARE cur CURSOR FOR
SELECT id, name, category, CONCAT(COALESCE(CONCAT(p_names, ' > '), ''), name) AS path
FROM item WHERE category <=> p_category;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur;
read_loop: LOOP
FETCH cur INTO v_id, v_name, v_category, v_path;
IF done THEN
LEAVE read_loop;
END IF;
SELECT v_id, v_name, v_category, v_path;
CALL get_item(v_id, v_path);
END LOOP;
CLOSE cur;
END//
DELIMITER ;
结果作为一系列17个不同的结果集返回，每个结果集各占一行：
mysql> CALL get_item(NULL, NULL);
+------+-----------+------------+-----------+
| v_id | v_name | v_category | v_path |
+------+-----------+------------+-----------+
| 1 | groceries | NULL | groceries |
+------+-----------+------------+-----------+
1 row in set (0.00 sec)
+------+--------+------------+--------------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+--------------------+
| 4 | grains | 1 | groceries > grains |
+------+--------+------------+--------------------+
1 row in set (0.00 sec)
+------+---------+------------+---------------------+
| v_id | v_name | v_category | v_path |
+------+---------+------------+---------------------+
| 5 | produce | 1 | groceries > produce |
+------+---------+------------+---------------------+
1 row in set (0.00 sec)
+------+--------+------------+-------------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+-------------------+
| 6 | meats | 1 | groceries > meats |
+------+--------+------------+-------------------+
1 row in set (0.00 sec)
+------+--------+------------+-------------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+-------------------+
| 7 | dairy | 1 | groceries > dairy |
+------+--------+------------+-------------------+
1 row in set (0.01 sec)
+------+--------+------------+--------------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+--------------------+
| 8 | snacks | 1 | groceries > snacks |
+------+--------+------------+--------------------+
1 row in set (0.01 sec)
+------+--------+------------+----------------------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+----------------------------+
| 14 | chips | 8 | groceries > snacks > chips |
+------+--------+------------+----------------------------+
1 row in set (0.01 sec)
+------+--------+------------+--------------------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+--------------------------+
| 15 | dip | 8 | groceries > snacks > dip |
+------+--------+------------+--------------------------+
1 row in set (0.01 sec)
+------+------------+------------+---------------------------------+
| v_id | v_name | v_category | v_path |
+------+------------+------------+---------------------------------+
| 16 | snack bars | 8 | groceries > snacks > snack bars |
+------+------------+------------+---------------------------------+
1 row in set (0.01 sec)
+------+-----------+------------+-----------------------+
| v_id | v_name | v_category | v_path |
+------+-----------+------------+-----------------------+
| 9 | beverages | 1 | groceries > beverages |
+------+-----------+------------+-----------------------+
1 row in set (0.01 sec)
+------+--------+------------+--------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+--------+
| 2 | retail | NULL | retail |
+------+--------+------------+--------+
1 row in set (0.01 sec)
+------+--------+------------+--------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+--------------+
| 10 | car | 2 | retail > car |
+------+--------+------------+--------------+
1 row in set (0.01 sec)
+------+--------+------------+---------------+
| v_id | v_name | v_category | v_path |
+------+--------+------------+---------------+
| 11 | home | 2 | retail > home |
+------+--------+------------+---------------+
1 row in set (0.02 sec)
+------+----------+------------+-------------------+
| v_id | v_name | v_category | v_path |
+------+----------+------------+-------------------+
| 12 | clothing | 2 | retail > clothing |
+------+----------+------------+-------------------+
1 row in set (0.02 sec)
+------+-------------+------------+----------------------+
| v_id | v_name | v_category | v_path |
+------+-------------+------------+----------------------+
| 13 | electronics | 2 | retail > electronics |
+------+-------------+------------+----------------------+
1 row in set (0.02 sec)
+------+---------+------------+---------+
| v_id | v_name | v_category | v_path |
+------+---------+------------+---------+
| 3 | service | NULL | service |
+------+---------+------------+---------+
1 row in set (0.02 sec)
+------+---------+------------+-------------------+
| v_id | v_name | v_category | v_path |
+------+---------+------------+-------------------+
| 17 | haircut | 3 | service > haircut |
+------+---------+------------+-------------------+
1 row in set (0.02 sec)
在应用程序代码中调用它时，需要将其作为多结果集语句循环。
我几乎没有在MySQL中使用存储过程。这在应用程序代码中会简单得多。
我也更喜欢使用替代方法来查询分层数据集，而不是使用递归。幸运的是，在MySQL 8.0中，您有递归查询（就像我们已经在几乎所有其他SQL数据库中一样）。您可以在MySQL 8.0中执行此操作，而无需使用存储过程：
WITH RECURSIVE MyTree AS (
SELECT id, name, category, name AS path FROM item WHERE category IS NULL
UNION ALL
SELECT i.id, i.name i.category, CONCAT(t.path, ' > ', i.name)
FROM item AS i JOIN MyTree AS t ON i.category = t.id
)
SELECT * FROM MyTree;