我有以下过程，旨在检测无向图中的循环，该图中采用边（单边）和边集（边集）。还有两个参数，left_set（用于存储要传递给递归的必要边）和 cyclic（它是一个布尔值，最终确定图是否是循环的。
由于某种原因，检测在第一次递归之后不起作用。这是带有解释详细信息的注释的代码：
我在 MYSQL 中实现了以下功能（以避免混淆）：
-concat_set()：如果是空集，则返回两个集的连接，以说明放错位置的 ','
-remove_first()：从集合中移除第一个成员
-get_left_node()/get_right_node：返回边的节点，边之间的分隔符是':'，所以边看起来像这样'12:15'
CREATE PROCEDURE `is_cyclic`(
IN `singleedge` VARCHAR(15),
IN `edgeset` VARCHAR(1024),
IN 'left_set' VARCHAR(512),
OUT `cyclic` BOOLEAN)
BEGIN
DECLARE se_left VARCHAR(5);
DECLARE es_left VARCHAR(5);
DECLARE se_right VARCHAR(5);
DECLARE es_right VARCHAR(5);
Call get_left_node(singleedge, se_left);
Call get_left_node(SUBSTRING_INDEX(edgeset, ',', 1), es_left);
Call get_right_node(singleedge, se_right);
Call get_right_node(SUBSTRING_INDEX(edgeset, ',', 1), es_right);
--is edgeset emptY?
IF LENGTH(edgeset)= 0 AND LENGTH(left_set) = 0 THEN
BEGIN
SET cyclic= false;
END;
--are singeeledge and first edge in edgeset the same?
ELSEIF ((se_left = es_left
OR se_left= es_right)
AND( se_right = es_left
OR se_right = es_right)) THEN
BEGIN
set cyclic= true;
END;
--do singleedge and first edge in edgeset share any vertices?
ELSEIF se_left = es_left
OR se_left= es_right
OR se_right = es_left
OR se_right = es_right
THEN
--check for all possiblities
BEGIN
--if left vertex of singleedge and left vertex of current edge in front of edgeset are the same
IF se_left=es_left THEN
BEGIN
--test if the edge of combined uncommon vertices (forming concat(se_right,':',es_right)) exists in the remaining edgeset concatanated with the left_set
CALL is_cyclic(concat(se_right,':',es_right),concat_set(left_set,remove_first(edgeset)), '', cyclic);
--if the recursion returns cyclic=false, then remove the considered edge from edgeset and continue trying to match the original singleedge with the rest of edgeset
IF cyclic=false THEN
CALL is_cyclic(singleedge, remove_first(edgeset), left_set, cyclic);
END IF;
END;
ELSEIF se_left= es_right THEN
BEGIN
CALL is_cyclic(concat(se_right,':',es_left), concat_set(left_set, remove_first(edgeset)), '', cyclic);
IF cyclic=false THEN
CALL is_cyclic(singleedge, remove_first(edgeset), left_set, cyclic);
END IF;
END;
ELSEIF se_right=es_left THEN
BEGIN
CALL is_cyclic(concat(se_left,':',es_right), concat_set(left_set, remove_first(edgeset)), '', cyclic);
IF cyclic=false THEN
CALL is_cyclic(singleedge, remove_first(edgeset), left_set, cyclic);
END IF;
END;
ELSE
BEGIN
CALL is_cyclic(concat(se_left,':',es_left), concat_set(left_set, remove_first(edgeset)), '', cyclic);
IF cyclic=false THEN
CALL is_cyclic(singleedge, remove_first(edgeset), left_set, cyclic);
END IF;
END;
END IF;
END;
ELSE
BEGIN
--if the current edge being considered from the edgeset does not contain any vertex in common with singleedge, set it aside into left_set and call is_cyclic recurisvely with the edge removed
SET left_set = concat_set(left_set, SUBSTRING_INDEX(edgeset, ',', 1));
CALL is_cyclic(singleedge, remove_first(edgeset), left_set, cyclic);
END;
END IF;
END