Question

I have a table which only contains id and a field whose data is a list of data. e.g.

--------------
| id |  data  |
| 1  | a,b,c,d|
| 2  | a,b,k,m|
---------------

I guess it’s not a good design that put a list data in a field, so I want to know how can I redesign it?

Answer 1

As per me you need two tables i.e. Master and Transaction tables only when some details are gonna be same for every records and some are gonna be changing. In your case if there are not any other thing related to your id field is gonna be same you can carry on with one table and with following structure.

--------------
| id |  data  |
| 1  |   a    |
| 1  |   b    |
| 1  |   c    |
| 1  |   d    |
| 2  |   a    |
| 2  |   b    |
| 2  |   k    |
| 2  |   m    |
---------------

BUT if there are any other things related to the id fields that is gonna be same for same id records you will have to use two tables.
like following case. there are 3 fields id, name and data.
and you current table looks something like

--------------------------
| id |  name    |  data  |
| 1  | testname | a,b,c,d|
| 2  | remy     | a,b,c,d|
--------------------------

your new table structure should look like.

table 1 Master

-----------------
| id |  name    |
| 1  | testname |
| 2  | remy     |
-----------------

Table 2 Transaction

    --------------
    | id |  data  |
    | 1  |   a    |
    | 1  |   b    |
    | 1  |   c    |
    | 1  |   d    |
    | 2  |   a    |
    | 2  |   b    |
    | 2  |   k    |
    | 2  |   m    |
    ---------------

For better database management we might need to normalize the data.
Database normalization is the process of organizing the fields and tables of a relational database to minimize redundancy and dependency. Normalization usually involves dividing large tables into smaller (and less redundant) tables and defining relationships between them. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships. You can find more on below links

1. 3 Normal Forms Database Tutorial

2. Database normalization