SQL-列出所有先决条件（如循环）

Question

(I'm using SQL Plus, although I don't think that would matter too much)

Okay my database structure looks similar to this:

CREATE TABLE MODULE (
  module_code       INT,
  PRIMARY KEY (module_code)
);

CREATE TABLE PREREQUISITES (
  module_code       INT,
  prerequisite_code VARCHAR(6),
  FOREIGN KEY (module_code) REFERENCES MODULE(module_code),
  FOREIGN KEY (prerequisite_code) REFERENCES MODULE(module_code)
);

So the data may look like this:

MODULE:
5
15
20
100

PREREQUISITES:
100  20
100  15
15   5

My question is, is there a way to list all prerequisites for a module (eg prerequisites of 100 is: 20, 15 & 5 (because 15 has a prerequisite of 5) or must this be done programmatically.

I'm also aware there could be circular references. Again I can think of a way to do this programmatically but is there a way to detect this using just SQL.

Answer 1

Actually, the RDBMS you are using maters completely for what you are trying to accomplish. You are storing your hierarchical data in an adjacency list model. Since you are using oracle , you can use CONNECT BY to traverse the tree:

SELECT * 
FROM PREREQUISITES
START WITH module_code = 100
CONNECT BY module_code = PRIOR prerequisite_code

• SQL Fiddle Demo

And here's a good article on the subject: http://explainextended.com/2009/09/28/adjacency-list-vs-nested-sets-oracle/

---

If duplicates and infinite loops are potential concerns, you can use nocylcle to prevent the infinite loops and a subquery to return the distinct results:

SELECT * 
FROM (SELECT DISTINCT module_code, prerequisite_code FROM PREREQUISITES) T
START WITH module_code = 100
CONNECT BY NOCYCLE module_code = PRIOR prerequisite_code

• More Fiddle