Database design problem - sql

Question

I have a problem creating a database schema for the following scenario:
(I’m not creating a dating site but just using this as an example)
A user logs on to a dating site and is given a multiple selection for the hair colour they’d like their date to have:
This is easy enough to model with the three tables below:
Tables:
User
{key}
HairColour
{key}
UserHairColour
{UserKey}
{HairColourKey}
However, the user also has the option to select ‘any’ which means they don’t care about hair colour and all hair colour should be included in the selection.
How do I give the user the ‘any’ option?
I could obviously select all hair colours and shove them into ’UserHairColour’ but what if I need to add a new hair colour in the future?

Answer 1

Absence of any records for this particular user in the UserHairColour table will indicate they do not care about the hair colour.
Absence of a decision indicates they have no preference. Obviously, it cannot mean they want their date to have no hair color at all.
I do not see here a need for a separate value or any extra table design. What you have allows you to achieve your goal in a simple way.
EDIT: As reaction to a proposed solution with ANY extra value.
The idea of "ANY" will conceptually interfere with the other selections. We are talking about presenting the user with a multitude of choices, ANY being one of them, and allowing them to select many. So the user can technically select ANY along with the other options, making it unclear what takes precedence - ANY or specific options. I believe the approach with simply no records as an indicator of ANY is clearer - it can only be interpreted one way. No records - no preferred values. You obviously cannot interpret it in the other way - no preferred value - user does not want this value to be present - this will make for transparent hair color which makes no sense. You can say it can mean no hairs at all, but I would suggest to have a separate option or a separate question for that already.

Answer 2

Given the example above, I would just add 'Any' or 'No Preference' as a selection and treat it as a specific hair color. This would work the best because if you did want to add more specific hair colors. Typically when I create new relational models I tend to add a -1 for the first key entry and keep the values for that row as my default go to ones.
This would be better practice than just dummy'ing it out with a temp table or query in my opinion.

Answer 3

This should be simple to achieve. If the user chooses "Any", you simply handle it on the query:
select
*
from
User
left join
UserHairColour on UserHairColour.UserId=User.UserId
where
(#hairpreference = 'Any' OR UserHairColour.HairColourId=#hairpreference)
If you can set the input var #hairpreference to null instead of 'Any', then it gets easier:
where
(UserHairColour.HairColourId=COALESCE(#hairpreference, UserHairColour.HairColourId))

Answer 4

Declare a temp table, fill it with the color values and query like this:
SELECT *
FROM UserHairColor
JOIN User
ON User.id = UserHairColor.UserID
WHERE HairColorKey IN
(
SELECT ColorKey
FROM #mytable
)
UNION ALL
SELECT *
FROM UserHairColor
JOIN User
ON User.id = UserHairColor.UserID
AND NOT EXISTS
(
SELECT NULL
FROM #mytable
)
This will select all users with requested hair colors, of all users at all if the table is empty.

Answer 5

If users can select any number of HairColours, I think, for consistency, it would be useful to do shove a record in UserHairColours for every colour. If users can select only one, one of which is 'any', then I favour New in town's solution.

Answer 6

Put (PersonID, HairColorPreference) in a table of its own. If someone has no preference, just don't record a row in that table.
Use views to put together people with preference with just that preference, and people with no preference with all hair colors.
BTW, what are you going to do with people whose preference is "anything but purple"?

Answer 7

As clearly you are not going to build a dating site you may make it clear are the other answers here fulfill your need or not. But my suggestion is to creat another table to tell if a user has selected any hair color of no hair color at all( sounds nonsense in your example but may have meaning in other situation).
By having following tables in your database you may accomplish this.
Users
HairColor
TypeOfColorSelection(1:Selected, 2:All, 3:Exclude, ...)
UserColorSelectionProfile(UserID, TypeOfColorSelection)
UserPreferredColor(UserID, HairColor)

Answer 8

If you want the hair color option to be mandatory then the no choice (empty set) option doesn't work.

Answer 9

This reminds me of the classic UK TV ads for Whiskas cat food. The strapline was originally,
Eight out of ten owners say their
cat prefers it
Later, it was changed to
Eight out of ten owners who expressed
a preference said their cat
prefers it
[The italics are mine.]
Clearly, the results are skewed when failing to show the difference between implicitly explicitly having no preference, otherwise why change a purrfectly good strapline for one that doesn't scan quite as well? QED ;)
My preference would be to use separate tables to model those who expressed a preference (along with the colour(s) they chose), those who expressed they had no preference and those who expressed no preference.
For a worked example, see How To Handle Missing Information Without Using NULL by Hugh Darwen.