Question

I have two tables which look like the following

tools:

id | part name
---------------
 0 | hammer
 1 | sickle
 2 | axe

people:

 personID | ownedTool1 | ownedTool2 | ownedTool3 ..... ownedTool20
 ------------------------------------------------------------------
    0     |    2       |     1      |     3     ... ...    0

I’m trying to find out how many people own a particular tool. A person cannot own multiple copies of the same tool.

The only way I can think of doing this is something like

SELECT COUNT(*)
FROM tools JOIN people ON tools.id = people.ownedTool1.id OR tools.id = people.ownedTool2 ... and so on
WHERE tools.id = 0

to get the number of people who own hammers. I believe this will work, however, this involves having 20 OR statements in the query. Surely there is a more appropriate way to form such a query and I’m interested to learn how to do this.

Answer 1

You shouldn’t have 20 columns each possibly containing an ID in the first place. You should properly establish a normalized schema. If a tool can belong to only one user – but a user can have multiple tools, you should establish a One to Many relationship. Each tool will have a user id in its row that maps back to the user it belongs to. If a tool can belong to one or more users you will need to establish a Many to Many relationship. This will require an intermediate table that contains rows of user_id to tool_id mappings. Having a schema set up appropriately like that will make the query you’re looking to perform trivial.

In your particular case it seems like a user can have many tools and a tool can be “shared” by many users. For your many-to-many relation all you would have to do is count the number of rows in that intermediate table having your desired tool_id.

Something like this:

SELECT COUNT(ID) FROM UserTools Where ToolID = @desired_tool_id

Googling the terms I bolded should get you pointed in the correct direction. If you’re stuck with that schema then the way you pointed out is the only way to do it.