教你如何使用MySQL8递归的方法

之前写过一篇 MySQL通过自定义函数的方式，递归查询树结构，从MySQL 8.0 开始终于支持了递归查询的语法

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 为我们定义的CTE，可以在当前查询中引用

可以看出 CTE 就是一个临时结果集，和派生表类似，二者的区别这里不细说，可以参考下MySQL开发文档：https://dev.mysql.com/doc/refman/8.0/en/with.html#common-table-expressions-recursive-examples

递归查询

先来看下递归查询的语法

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

递归部分不能包括：

上面的讲解可能有点抽象，通过例子慢慢来理解

WITH RECURSIVE cte (n) AS -- 这里定义的n相当于结果集的列名，也可在下面查询中定义( SELECT 1 UNION ALL SELECT n + 1 FROM cte WHERE n < 5)SELECT * FROM cte;-- result+------+| n  |+------+|  1 ||  2 ||  3 ||  4 ||  5 |+------+

EXAMPLE

最后来看一个树结构的例子

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 |+----+---------+-----------+