在定义外键可以对外键进行update 和delete 时对字表进行的操作
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    1. mysql> create table parent(
    2.     -> id int primary key);
    3. Query OK, 0 rows affected (0.01 sec)
    5. mysql> create table child (id int,parent_id int,foreign key(parent_id) references parent(id) on delete set null);
    6. Query OK, 0 rows affected (0.02 sec)
    8. mysql> insert into parent values (1),(2),(3);
    9. Query OK, 3 rows affected (0.00 sec)
    10. Records: 3  Duplicates: 0  Warnings: 0
    12. mysql> insert into child values(1,1),(2,2),(3,3);
    13. Query OK, 3 rows affected (0.01 sec)
    14. Records: 3  Duplicates: 0  Warnings: 0
    16. mysql> delete from parent where id=1;
    17. Query OK, 1 row affected (0.01 sec)
    19. mysql> select * from child;
    20. +------+-----------+
    21. | id   | parent_id |
    22. +------+-----------+
    23. |    1 |      NULL |
    24. |    2 |         2 |
    25. |    3 |         3 |
    26. +------+-----------+
    27. 3 rows in set (0.00 sec)
    29. mysql>