Query to find the previous activity in STM

I have a data base to reflect a State of Transition (STM) Machine. It contains 3 tables: activity, node, and Transition.

Tables of knots and Transition are the configuration of the STM. They define the different possible States and the possible transitions between States.

The way that I take through the STM is recorded in the activity Table. A record is created for each visited node.

I need a query that uses the id of my current activity to find my previous activity, i.e. the activity that precedes me directly. The following rules apply:
-My Id is meaningless
-It should work for different configurations of the transition and the node tables, but:
-The starting node is always the node "A".
-There is no recursive transition, for example ('B', 'B');
-There is no circular transitions, for example ('B', 'C') and ('C', 'B')
-Transition and node tables will never change between the creation of the first activity and the execution of the query.
-The path reflected in the activity table is always a valid path.
-My current activity is always the last activity.

For all data below, find the previous activity of activity with id = 4. The correct answer is "C".
DROP TABLE Transition;
DROP TABLE Activity;
DROP TABLE Node;

CREATE TABLE Node
(
     Id VARCHAR2(1) PRIMARY KEY
);

CREATE TABLE Activity
(
     Id Number(8,0) PRIMARY KEY,
     Node_Id VARCHAR2(1)
);

CREATE TABLE Transition
(
     FromNode_Id VARCHAR2(1),
     ToNode_Id VARCHAR2(1)
);

ALTER TABLE Activity
ADD FOREIGN KEY
(Node_Id) REFERENCES Node(Id);

ALTER TABLE Transition
ADD FOREIGN KEY
(FromNode_Id) REFERENCES Node(Id);

ALTER TABLE Transition
ADD FOREIGN KEY
(ToNode_Id) REFERENCES Node(Id);

INSERT INTO Node VALUES ('A');
INSERT INTO Node VALUES ('B');
INSERT INTO Node VALUES ('C');
INSERT INTO Node VALUES ('D');

INSERT INTO Transition VALUES ('A','B');
INSERT INTO Transition VALUES ('B','C');
INSERT INTO Transition VALUES ('B','D');
INSERT INTO Transition VALUES ('C','D');

INSERT INTO Activity VALUES (1,'A');
INSERT INTO Activity VALUES (2,'B');
INSERT INTO Activity VALUES (3,'C');
INSERT INTO Activity VALUES (4,'D');
Desired output:
ID
-
C

Hello

Assuming that all Activity_id tells us nothing on the way, but all lines of activity together to form a single path, we can get results this way:

WITH     all_paths    AS
(
     SELECT     'A' || SYS_CONNECT_BY_PATH (a.Node_id, '/')
              || '/'     AS node_id_path
     FROM     Transition  t
     JOIN     Activity    a  ON  a.Node_Id  = t.ToNode_Id
     WHERE     LEVEL             = (
                            SELECT  COUNT (*)
                         FROM     activity
                         ) - 1
     START WITH  t.FromNode_Id  = 'A'
     CONNECT BY  t.FromNode_Id  = PRIOR t.ToNode_Id
)
SELECT       REGEXP_SUBSTR ( node_id_path
                 , '([^/]+)/' || (
                                    SELECT  Node_Id
                             FROM    activity
                             WHERE   Id     = 4  -- target_id
                                )
                         || '/'
               , 1
               , 1
               , NULL
               , 1
               )     AS prev_id
FROM       all_paths
;

This means, there is some character (I used "/" above) that we know never appears in Activity.Node_Id.

Since there are no loops in Transition, it cannot be more than 1 way that involves all activity lines. If there are N lines in operation, this full path will be one that extends to LEVEL = N - 1. (It is not N, N - 1, because the join between the activity and the Transition will always leave 1 line in activity with Node_Id = 'A'). As soon as we have the full path (which is node_id_path in 1 row, produced by all_paths), we just need to guess what was the Node_Id, which corresponds to the id of the target (4 in this example) and find the previous of the delimited node_id_path node_id.

Tags: Database

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