How to update on cascade in MySQL?

Question

Let\'s look at this example database:

As we can see, person depends on the city (person.ci

Answer 1

Here's a solution that uses cascading foreign keys to do what you describe:

mysql> create table city (
  id int not null auto_increment, 
  name varchar(45), 
  active tinyint, 
  primary key (id),
  unique key (id, active));

mysql> create table person (
  id int not null auto_increment, 
  city_id int,
  active tinyint, 
  primary key (id), 
  foreign key (city_id, active) references city (id, active) on update cascade);

mysql> insert into city (name, active) values ('New York', 1);

mysql> insert into person (city_id, active) values (1, 1);

mysql> select * from person;
+----+---------+--------+
| id | city_id | active |
+----+---------+--------+
|  1 |       1 |      1 |
+----+---------+--------+

mysql> update city set active = 0 where id = 1;

mysql> select * from person;
+----+---------+--------+
| id | city_id | active |
+----+---------+--------+
|  1 |       1 |      0 |
+----+---------+--------+

Tested on MySQL 5.5.31.

Answer 2

Maybe you should reconsider how you define a person to be active.. Instead of having active defined twice, you should just keep it in the city table and have your SELECT statements return Person WHERE city.active = 1..

But if you must.. you could do something like:

UPDATE city C
LEFT JOIN person P ON C.id = P.city
SET C.active = 0 AND P.active = 0
WHERE C.id = @id