子-MYsql中的父约束？

Question

Can I have them?

I have a constraint that forces delete of the row if a related row from a different table is deleted:

node INT UNSIGNED NOT NULL,
id INT UNSIGNED NOT NULL AUTO_INCREMENT,   
name VARCHAR(20) NOT NULL,     
title VARCHAR(200) NOT NULL,
description MEDIUMTEXT,
category VARCHAR(20) NOT NULL,  
parent INT UNSIGNED DEFAULT 0,             # here is the parent term id
                                           # if it's 0 the term has no parent

PRIMARY KEY(id),
UNIQUE KEY(name, node, category),
KEY parent(parent),
KEY category(category),

CONSTRAINT terms FOREIGN KEY(node)
    REFERENCES nodes(id) ON DELETE CASCADE ON UPDATE RESTRICT

I want another constraint that forces auto-delete of the row if the "parent" term is deleted? (if there is a term parent set)

Example:

node  |  id  |  name  | title   | description    | category | parent
___________________________________________________________________
534   |    1 |  A     | Foooooo | Bla bla...     | A        | 0 
54    |    2 |  B     | Foooooo | Bla bla...     | A        | 1
45    |    3 |  C     | Foooooo | Bla bla...     | A        | 2
545   |    4 |  D     | Foooooo | Bla bla...     | A        | 2
534   |    5 |  E     | Foooooo | Bla bla...     | A        | 1

so terms look like:

  A
  - B
  --- C
  --- D
  - E

So if I delete B, I want C and D to be deleted as well, if I delete A, then B, C, D, E should be deleted...

Answer 1

There is no such thing as a "conditional" constraint: Either your parent field allways references another row (ie no 0 allowed), or it is no constraint at all.

This is quite a normal usecase, typically you would work around it with an ON DELETE TRIGGER on the parent, that also deletes the children, selecting them by parent.

In your example you might consider something like

CREATE TRIGGER cascade_delete_children 
    BEFORE DELETE ON node 
    FOR EACH ROW 
         DELETE FROM node WHERE parent=OLD.id;

Answer 2

You can use NULL instead of 0 and define the FOREIGN KEY with cascading deletes:

parent INT UNSIGNED NULL DEFAULT NULL,    --- here is the parent term id
                                          --- if it's NULL the term has no parent
...

CONSTRAINT parent_fk FOREIGN KEY(parent)
    REFERENCES thisTable(id) 
        ON DELETE CASCADE 
        ON UPDATE RESTRICT