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
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
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;
RECURSIVE
代表当前 CTE 是递归的SELECT * FROM cte;
拿到所有的结果集递归部分不能包括:
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 |+------+
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 |+----+---------+-----------+
select * from c_tree where parent_id = 3
inner join
得到 R1inner join
得到 R2https://dev.mysql.com/doc/refman/8.0/en/with.html
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