使用来自另一个表的concat更新MYSQL中的值

Question

I've been trying to update values of a field from another table's field with comma separators.

This is my current code:

update programs set registered = (select user_id from users where reg_prog != 0)

I am getting #1242 mysql error.

I would like to have a list like this: Registered users: 47, 97, 10, 618, 5 (these are the ids of the users that are registered to the program).

Any help?

Answer 1

Don't do it like this. Use a normalized data model . Having any kind of comma-separated list in a database field usually is a very bad idea. You have no way of properly working with these values, because you always need messy string manipulation to do anything with this.

If you need to record the users that registered to a program, create separate tables for users, programs and a relation table for both of them:

 _________          ______________          _________
| program |        | user2program |        | program |
+---------+        +------------  +        +---------+
| id      |  <-->  | user_id      |        | name    |
| name    |        | program_id   |  <-->  | id      |
 ---------          --------------          ---------

It takes a little more programming effort, to keep these tables filled properly, but you will see that it works much better this way.

There is no single SQL statement that can do what you are looking for in your question. If you have a proper data model, you need to first insert your program, then your user and afterwards add the releation between the two:

INSERT INTO program (id, name) VALUES (1, "My Program");
INSERT INTO user (id, name) VALUES (1, "John Doe");
INSERT INTO user2program (user_id, program_id) VALUES (1, 1);

To get all users that are registered for a program , you can then use a SELECT statement with JOINs:

SELECT p.name AS program_name, u.name AS user_name
FROM   program AS p
JOIN   user2program AS u2p ON p.id = u2p.program_id
JOIN   user AS u ON u.id = u2p.user_id
WHERE  p.id = 1 -- ID of the program