SQL Unique Value With Primary & Foreign Keys

Question

I'm pretty sure this question has been asked before but using SQL I'm trying to only allow a table to have a unique value using both a primary and foreign keys. This may sound confusing so these are my SQL statements.

Folder table:

CREATE TABLE [dbo].[Folder] 
(
    [FolderID] VARCHAR(50) NOT NULL,
    [FolderURL] NCHAR(255) NOT NULL,
    [FolderTag] VARCHAR(50) NOT NULL,

    PRIMARY KEY CLUSTERED ([FolderURL] ASC),
    UNIQUE NONCLUSTERED ([FolderID] ASC)
);

Extension table:

CREATE TABLE [dbo].[Extension] 
(
    [ExtensionID] VARCHAR(10) NOT NULL,
    [FolderID] VARCHAR(50) NOT NULL,
    [LocationURL] VARCHAR(MAX) NOT NULL,

    PRIMARY KEY CLUSTERED ([ExtensionID] ASC),

    CONSTRAINT [fk_FolderID] 
        FOREIGN KEY ([FolderID])
        REFERENCES [dbo].[Folder]([FolderID]) ON DELETE CASCADE,
    CONSTRAINT uc_Extension UNIQUE ([ExtensionID], [FolderID]) 
);

In the Extension table I would like to have it have a unique value using the [FolderID] & [ExtensionID]. I first tried to achieve this by creating a primary key using both values, then I tried the current statement above.

Edit

I didn't give enough detail for what I am asking, below is a layout of both tables that i want to create and what the results are using the methods above.

Folder Table:

----------------------------------------------------------
| Folder ID |        FolderURL        |  FolderTag       |
|-----------|-------------------------|------------------|
|   Home    | C:/users/test/          | Home Folder      |
| Downloads | E:/Donwloads            | Downloads Folder |
|   Music   | C:/users/test/music     | Music Folder     |
| Documents | C:/users/test/documents | Downloads Folder |
----------------------------------------------------------

Extension Table:

---------------------------------------------------------
| Extension ID | FolderID  |        LocationURL         |
|--------------|-----------|----------------------------|
|    .jpg      | Downloads | C:/users/test/pictures     | = Pass 
|    .png      | Downloads | C:/users/test/pictures     | = Pass
|    .docx     | Documents | C:/users/test/BusinessDocs | = Pass
|    .mp3      |   Home    | C:/users/test/music        | = Pass
|    .jpg      |   Home    | C:/users/test/pictures     | = Fail
|    .mp3      | Downloads | C:/users/test/music        | = Fail
---------------------------------------------------------

From the table layouts provided above you can see that the last two fail due to the .mp3 ID is already entered into the table. what I am asking about is the to create a table which has a unique primary key, or something else which will allow for a unique ExtensionID and FolderID so it will allow the last two values in the Extension Table.

If anyone could help me I would greatly appreciate it.

Answer 1

CREATE TABLE [dbo].[Extension] 
(
    [ExtensionID] VARCHAR(10) NOT NULL,
    [FolderID] VARCHAR(50) NOT NULL,
    [LocationURL] VARCHAR(MAX) NOT NULL,

    PRIMARY KEY ([ExtensionID], [FolderID]),

    CONSTRAINT [fk_FolderID] 
        FOREIGN KEY ([FolderID])
        REFERENCES [dbo].[Folder]([FolderID]) ON DELETE CASCADE
);

---

Here's full code and output. SQLFiddle.com is down for me; this is in PostgreSQL syntax.

CREATE TABLE Folder
(
    FolderID VARCHAR(50) NOT NULL,
    FolderURL NCHAR(255) NOT NULL,
    FolderTag VARCHAR(50) NOT NULL,

    PRIMARY KEY (FolderURL),
    UNIQUE (FolderID)
);

CREATE TABLE Extension 
(
    ExtensionID VARCHAR(10) NOT NULL,
    FolderID VARCHAR(50) NOT NULL,
    LocationURL VARCHAR(255) NOT NULL,

    PRIMARY KEY (ExtensionID, FolderID),

    CONSTRAINT fk_FolderID 
        FOREIGN KEY (FolderID)
        REFERENCES Folder(FolderID) ON DELETE CASCADE
);

insert into folder values
('Home', 'C:/users/test/', 'Home Folder'),
('Downloads', 'E:/Donwloads', 'Downloads Folder'),
('Music', 'C:/users/test/music', 'Music Folder'),
('Documents', 'C:/users/test/documents', 'Downloads Folder');

insert into Extension values
('.jpg', 'Downloads', 'C:/users/test/pictures'),
('.png', 'Downloads', 'C:/users/test/pictures'),
('.docx', 'Documents', 'C:/users/test/BusinessDocs'),
('.mp3', 'Home', 'C:/users/test/music'),
('.jpg', 'Home', 'C:/users/test/pictures'),
('.mp3', 'Downloads', 'C:/users/test/music');

select * from folder;

folderid    folderurl                 foldertag
--
Home        C:/users/test/            Home Folder
Downloads   E:/Donwloads              Downloads Folder
Music       C:/users/test/music       Music Folder
Documents   C:/users/test/documents   Downloads Folder

select * from extension;

extensionid  folderid    locationurl
--
.jpg         Downloads   C:/users/test/pictures
.png         Downloads   C:/users/test/pictures
.docx         Documents  C:/users/test/BusinessDocs
.mp3         Home        C:/users/test/music
.jpg         Home        C:/users/test/pictures
.mp3         Downloads   C:/users/test/music