C:\>sqlite3 tree.db
SQLite version 3.6.19
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> create table tree(
   ...>         id integer not null primary key,
   ...>         parent_id integer null references tree(id) on delete cascade
   ...> );
sqlite> insert into tree values(0, null);
sqlite> insert into tree values(1, 0);
sqlite> insert into tree values(2, 0);
sqlite> insert into tree values(3, 1);
sqlite> insert into tree values(4, 1);
sqlite> insert into tree values(5, 2);
sqlite> select * from tree;
0|
1|0
2|0
3|1
4|1
5|2
sqlite> delete from tree where id=0;
sqlite> select * from tree;
1|0
2|0
3|1
4|1
5|2
sqlite>

sqlite> PRAGMA foreign_keys = ON;
sqlite> insert into tree values(6, 7);
SQL error: foreign key constraint failed
sqlite> insert into tree values(6, 3);
sqlite> select * from tree;
1|0
2|0
3|1
4|1
5|2
6|3
sqlite> delete from tree where id=1;
sqlite> select * from tree;
2|0
5|2
sqlite>