在 MySQL 字段中存储 csv – 坏主意？

Question

I have two tables, one user table and an items table. In the user table, there is the field "items". The "items" table only consists of a unique id and an item_name.

Now each user can have multiple items. I wanted to avoid creating a third table that would connect the items with the user but rather have a field in the user_table that stores the item ids connected to the user in a "csv" field.

So any given user would have a field "items" that could have a value like "32,3,98,56".

It maybe is worth mentioning that the maximum number of items per user is rather limited (<5).

The question: Is this approach generally a bad idea compared to having a third table that contains user->item pairs?

Wouldn't a third table create quite an overhead when you want to find all items of a user (I would have to iterate through all elements returned by MySQL individually).

Answer 1

You don't want to store the value in the comma separated form.

Consider the case when you decide to join this column with some other table.

Consider you have,

x  items
1  1, 2, 3
1  1, 4
2  1

and you want to find distinct values for each x ie:

x  items
1  1, 2, 3, 4
2  1

or may be want to check if it has 3 in it

or may be want to convert them into separate rows:

x  items
1  1
1  2
1  3
1  1
1  4
2  1

It will be a HUGE PAIN.

Use atleast normalization 1st principle - have separate row for each value.

Now, say originally you had this as you table:

x  item
1  1
1  2
1  3
1  1
1  4
2  1

You can easily convert it into csv values:

select x, group_concat(item order by item) items
from t
group by x

If you want to search if x = 1 has item 3. Easy.

select * from t where x = 1 and item = 3

which in earlier case would use horrible find_in_set :

select * from t where x = 1 and find_in_set(3, items);

If you think you can use like with CSV values to search, then first like %x% can't use indexes. Second, it will produce wrong results.
Say you want check if item ab is present and you do %ab% it will return rows with abc abcd abcde .... .

If you have many users and items, then I'd suggest create separate table users with an PK userid, another items with PK itemid and lastly a mapping table user_item having userid, itemid columns.

If you know you'll just need to store and retrieve these values and not do any operation on it such as join, search, distinct, conversion to separate rows etc. etc. - may be just may be, you can (I still wouldn't).

Answer 2

Storing complex data directly in a relational database is a nonstandard use of a relational database. Normally they are designed for normalized data.

There are extensions which vary according to the brand of software which may help. Or you can normalize your CSV file into properly designed table(s). It depends on lots of things. Talk to your enterprise data architect in this case.

Whether it's a bad idea depends on your business needs. I can't assess your business needs from way out here on the internet. Talk to your product manager in this case.