在一列中存储来自另一个表的多个ID？

Question

Apologies if this is really stupid but I don't have any experience in php and mysql to know how things should be done. I have a customer table in a mysql db and a group table:

customers - ID name email phone group groups - ID name description

So I need to assign groups to customers if necessary, this can be more than one group to each customer. So eg customer 1 is in group 4,5,6

What way should I assign groups in the group column of the customer table. Should I just add the group ID's separated by commas, then just use explode when I need to get the individual ID's out?

Maybe this isn't the right approach at all, could someone enlighten me please. I would appreciate knowing the right way to do this, thanks.

Answer 1

Do not store multiple IDs in one column. This is a denormalization that will make it much harder to query and change your data, as well as hurting performance.

Instead, create a separate CustomerGroup table (with CustomerID and GroupID columns), and have one row per Customer/Group relationship.

Answer 2

Should I just add the group ID's separated by commas, then just use explode when I need to get the individual ID's out?

No! If you do that then you won't quickly be able to (for example) query for which users there are in a specific group.

Instead use a join table with two columns, each of which has a foriegn key constraint to the corresponding table.

group_id  customer_id
4         1
5         1
6         1

Answer 3

Here is an example of tables to show how you should implement this :

Table 1 CONSUMERS:

id name email 
1  john john@something.com
2  ray  ray@something.com

Table 2 GROUPS :

id group_name   description
1  music        good music group
2  programming  programmers

Table 3 CONSUMERS_GROUPS

consumer_id  group_id
1              1
1              2
2              1

Now the table 3 is listing consumers ids which belong to which group id.

This type of relationship is called one to many relation where, one consumer can have many groups. Reverse might also be true where one group can have many consumers. In that case relationship is called many to many