MySQL : Automatic column update with list of value - mysql

Question

I just want to update automatically a column of 2 tables linked with foreign keys.
To more, these columns have to contain not just one value, but a list of values.
Here is my example :
Table Name : members
Member name : Demo User
Assigned Tasks : <List of tasks assigned to "Demo User">
Table Name : tasks
Task Name : Modeling
Assigned To : <List of users assigned to "Modeling">
In each table, I want to be able to insert value in the column, and the other table is automatically updated.
Example :
Table Name : members
Member name : Demo User
Assigned Tasks : Modeling,Mapping
Table Name : tasks
Task Name : Modeling
Assigned To : AUTOMATIC UPDATE ->Demo User
Task Name : Mapping
Assigned To : AUTOMATIC UPDATE ->Demo User
I tried with foreign keys (using innoDB), but I just succeed to insert one value in the column, and the other table was not updated automatically...
Thank you for your help !

Answer 1

Never, ever store delimited values in a database. Normalize your data by creating a many-to-many table (in your case let's call it assigned_tasks) instead. It will pay off big time in a long run by enabling you to normally maintain and query your data.
That being said your schema might look like
CREATE TABLE members
(
member_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
member_name VARCHAR(32)
);
CREATE TABLE tasks
(
task_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
task_name VARCHAR(32)
);
CREATE TABLE assigned_tasks
(
member_id int,
task_id int,
PRIMARY KEY (member_id, task_id),
FOREIGN KEY (member_id) REFERENCES members (member_id),
FOREIGN KEY (task_id) REFERENCES tasks (task_id)
);
Now if you need a representation with a delimited list of tasks per user you can use GROUP_CONCAT() and GROUP BY
SELECT m.member_id, m.member_name, GROUP_CONCAT(t.task_name) tasks
FROM assigned_tasks a JOIN members m
ON a.member_id = m.member_id JOIN tasks t
ON a.task_id = t.task_id
GROUP BY m.member_id, m.member_name
Output:
| MEMBER_ID | MEMBER_NAME | TASKS |
|-----------|-------------|------------------|
| 1 | Demo User | Modeling,Mapping |
Here is SQLFiddle demo