Database design for user settings

Question

Other answers have ably outlined the pros and cons of your various options.

I believe that your Option 1 (property bag) is the best overall design for most applications, especially if you build in some protections against the weaknesses of propety bags.

See the following ERD:

Property Bag ERD

In the above ERD, the USER_SETTING table is very similar to OP’s. The difference is that instead of varchar Code and Value columns, this design has a FK to a SETTING table which defines the allowable settings (Codes) and two mutually exclusive columns for the value. One option is a varchar field that can take any kind of user input, the other is a FK to a table of legal values.

The SETTING table also has a flag that indicates whether user settings should be defined by the FK or by unconstrained varchar input. You can also add a data_type to the SETTING to tell the system how to encode and interpret the USER_SETTING.unconstrained_value. If you like, you can also add the SETTING_GROUP table to help organize the various settings for user-maintenance.

This design allows you to table-drive the rules around what your settings are. This is convenient, flexible and easy to maintain, while avoiding a free-for-all.

EDIT: A few more details, including some examples…

Note that the ERD, above, has been augmented with more column details (range values on SETTING and columns on ALLOWED_SETTING_VALUE).

Here are some sample records for illustration.

SETTING:
+----+------------------+-------------+--------------+-----------+-----------+
| id | description      | constrained | data_type    | min_value | max_value |
+----+------------------+-------------+--------------+-----------+-----------+
| 10 | Favourite Colour | true        | alphanumeric | {null}    | {null}    |
| 11 | Item Max Limit   | false       | integer      | 0         | 9001      |
| 12 | Item Min Limit   | false       | integer      | 0         | 9000      |
+----+------------------+-------------+--------------+-----------+-----------+

ALLOWED_SETTING_VALUE:
+-----+------------+--------------+-----------+
| id  | setting_id | item_value   | caption   |
+-----+------------+--------------+-----------+
| 123 | 10         | #0000FF      | Blue      |
| 124 | 10         | #FFFF00      | Yellow    |
| 125 | 10         | #FF00FF      | Pink      |
+-----+------------+--------------+-----------+

USER_SETTING:
+------+---------+------------+--------------------------+---------------------+
| id   | user_id | setting_id | allowed_setting_value_id | unconstrained_value |
+------+---------+------------+--------------------------+---------------------+
| 5678 | 234     | 10         | 124                      | {null}              |
| 7890 | 234     | 11         | {null}                   | 100                 |
| 8901 | 234     | 12         | {null}                   | 1                   |
+------+---------+------------+--------------------------+---------------------+

From these tables, we can see that some of the user settings which can be determined are Favourite Colour, Item Max Limit and Item Min Limit. Favourite Colour is a pick list of alphanumerics. Item min and max limits are numerics with allowable range values set. The SETTING.constrained column determines whether users are picking from the related ALLOWED_SETTING_VALUEs or whether they need to enter a USER_SETTING.unconstrained_value. The GUI that allows users to work with their settings needs to understand which option to offer and how to enforce both the SETTING.data_type and the min_value and max_value limits, if they exist.

Using this design, you can table drive the allowable settings including enough metadata to enforce some rudimentary constraints/sanity checks on the values selected (or entered) by users.

EDIT: Example Query

Here is some sample SQL using the above data to list the setting values for a given user ID:

-- DDL and sample data population...
CREATE TABLE SETTING
    (`id` int, `description` varchar(16)
     , `constrained` varchar(5), `data_type` varchar(12)
     , `min_value` varchar(6) NULL , `max_value` varchar(6) NULL)
;

INSERT INTO SETTING
    (`id`, `description`, `constrained`, `data_type`, `min_value`, `max_value`)
VALUES
    (10, 'Favourite Colour', 'true', 'alphanumeric', NULL, NULL),
    (11, 'Item Max Limit', 'false', 'integer', '0', '9001'),
    (12, 'Item Min Limit', 'false', 'integer', '0', '9000')
;

CREATE TABLE ALLOWED_SETTING_VALUE
    (`id` int, `setting_id` int, `item_value` varchar(7)
     , `caption` varchar(6))
;

INSERT INTO ALLOWED_SETTING_VALUE
    (`id`, `setting_id`, `item_value`, `caption`)
VALUES
    (123, 10, '#0000FF', 'Blue'),
    (124, 10, '#FFFF00', 'Yellow'),
    (125, 10, '#FF00FF', 'Pink')
;

CREATE TABLE USER_SETTING
    (`id` int, `user_id` int, `setting_id` int
     , `allowed_setting_value_id` varchar(6) NULL
     , `unconstrained_value` varchar(6) NULL)
;

INSERT INTO USER_SETTING
    (`id`, `user_id`, `setting_id`, `allowed_setting_value_id`, `unconstrained_value`)
VALUES
    (5678, 234, 10, '124', NULL),
    (7890, 234, 11, NULL, '100'),
    (8901, 234, 12, NULL, '1')
;

And now the DML to extract a user’s settings:

-- Show settings for a given user
select
  US.user_id 
, S1.description 
, S1.data_type 
, case when S1.constrained = 'true'
  then AV.item_value
  else US.unconstrained_value
  end value
, AV.caption
from USER_SETTING US
  inner join SETTING S1
    on US.setting_id = S1.id 
  left outer join ALLOWED_SETTING_VALUE AV
    on US.allowed_setting_value_id = AV.id
where US.user_id = 234

See this in SQL Fiddle.

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