之前寫過一篇 MySQL通過自定義函數的方式，遞歸查詢樹結構，從MySQL 8.0 開始終于支持了遞歸查詢的語法

CTE

首先了解一下什么是 CTE，全名 Common Table Expressions

WITH cte1 AS (SELECT a, b FROM table1), cte2 AS (SELECT c, d FROM table2)SELECT b, d FROM cte1 JOIN cte2WHERE cte1.a = cte2.c;

cte1, cte2 為我們定義的CTE，可以在當前查詢中引用

可以看出 CTE 就是一個臨時結果集，和派生表類似，二者的區別這里不細說，可以參考下MySQL開發文檔：https://dev.mysql.com/doc/refman/8.0/en/with.html#common-table-expressions-recursive-examples

遞歸查詢

先來看下遞歸查詢的語法

WITH RECURSIVE cte_name AS(  SELECT ...   -- return initial row set  UNION ALL / UNION DISTINCT  SELECT ...   -- return additional row sets)SELECT * FROM cte;

遞歸部分不能包括：

上面的講解可能有點抽象，通過例子慢慢來理解

WITH RECURSIVE cte (n) AS -- 這里定義的n相當于結果集的列名，也可在下面查詢中定義( SELECT 1 UNION ALL SELECT n + 1 FROM cte WHERE n < 5)SELECT * FROM cte;-- result+------+| n  |+------+|  1 ||  2 ||  3 ||  4 ||  5 |+------+

EXAMPLE

最后來看一個樹結構的例子

CREATE TABLE `c_tree` ( `id` int(11) NOT NULL AUTO_INCREMENT, `cname` varchar(255) COLLATE utf8mb4_unicode_ci DEFAULT NULL, `parent_id` int(11) DEFAULT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

mysql> select * from c_tree;+----+---------+-----------+| id | cname  | parent_id |+----+---------+-----------+| 1 | 1    |     0 || 2 | 2    |     0 || 3 | 3    |     0 || 4 | 1-1   |     1 || 5 | 1-2   |     1 || 6 | 2-1   |     2 || 7 | 2-2   |     2 || 8 | 3-1   |     3 || 9 | 3-1-1  |     8 || 10 | 3-1-2  |     8 || 11 | 3-1-1-1 |     9 || 12 | 3-2   |     3 |+----+---------+-----------+

mysql> WITH RECURSIVE tree_cte as(  select * from c_tree where parent_id = 3  UNION ALL  select t.* from c_tree t inner join tree_cte tcte on t.parent_id = tcte.id)SELECT * FROM tree_cte;+----+---------+-----------+| id | cname  | parent_id |+----+---------+-----------+| 8 | 3-1   |     3 || 12 | 3-2   |     3 || 9 | 3-1-1  |     8 || 10 | 3-1-2  |     8 || 11 | 3-1-1-1 |     9 |+----+---------+-----------+