Database design for user settings

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:

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 userselect  US.user_id , S1.description , S1.data_type , case when S1.constrained = 'true'  then AV.item_value  else US.unconstrained_value  end value, AV.captionfrom 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.idwhere US.user_id = 234

See this in SQL Fiddle.

Option 1 (as noted, "property bag") is easy to implement - very little up-front analysis. But it has a bunch of downsides.

1. If you want to restrain the valid values for UserSettings.Code, you need an auxiliary table for the list of valid tags. So you have either (a) no validation on UserSettings.Code – your application code can dump any value in, missing the chance to catch bugs, or you have to add maintenance on the new list of valid tags.

2. UserSettings.Value probably has a string data type to accommodate all the different values that might go into it. So you have lost the true data type – integer, Boolean, float, etc., and the data type checking that would be done by the RDMBS on insert of an incorrect values. Again, you have bought yourself a potential QA problem. Even for string values, you have lost the ability to constrain the length of the column.

3. You cannot define a DEFAULT value on the column based on the Code. So if you wanted EmailLimitMax to default to 5, you can’t do it.

4. Similarly, you can’t put a CHECK constraint on the Values column to prevent invalid values.

5. The property bag approach loses validation of SQL code. In the named column approach, a query that says “select Blah from UserSettings where UserID = x” will get a SQL error if Blah does not exist. If the SELECT is in a stored procedure or view, you will get the error when you apply the proc/view – way before the time the code goes to production. In the property bag approach, you just get NULL. So you have lost another automatic QA feature provided by the database, and introduced a possible undetected bug.

6. As noted, a query to find a UserID where conditions apply on multiple tags becomes harder to write – it requires one join into the table for each condition being tested.

7. Unfortunately, the Property Bag is an invitation for application developers to just stick a new Code into the property bag without analysis of how it will be used in the rest of application. For a large application, this becomes a source of “hidden” properties because they are not formally modeled. It’s like doing your object model with pure tag-value instead of named attributes: it provides an escape valve, but you’re missing all the help the compiler would give you on strongly-typed, named attributes. Or like doing production XML with no schema validation.

8. The column-name approach is self-documenting. The list of columns in the table tells any developer what the possible user settings are.

I have used property bags; but only as an escape valve and I have often regretted it. I have never said “gee, I wish I had made that explicit column be a property bag.”

Consider this simple example.

If you have 2 tables, UserTable(contains user details) and SettingsTable(contains settings details). Then create a new table UserSettings for relating the UserTable and SettingsTable as shown below

Hope you will found the right solution from this example.