SQL在同一表上的关系

Question

I haven't ever really used foreign keys in my databases, I have always just written code that would enforce things for me, but I'm looking towards shifting some of that logic to the database and learning more about foreign keys.

I currently have a database which has the table Categories :

CREATE TABLE [dbo].[Categories](
    [CategoryID] [bigint] IDENTITY(1,1) NOT NULL,
    [ParentCategoryID] [bigint] NOT NULL,
    [CategoryStatus] [bit] NOT NULL,
    [CategoryTitle] [varchar](64) NOT NULL,
    [CategorySlug] [varchar](64) NOT NULL,
    [CategoryThumbnail] [varchar](128) NULL,
    [CategoryHeaderImage] [varchar](128) NULL,
    [CategoryDescription] [text] NULL,
 CONSTRAINT [PK_Categories] PRIMARY KEY CLUSTERED 
(
    [CategoryID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [UI_Categories_Slug] UNIQUE NONCLUSTERED 
(
    [CategorySlug] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

I would like to create a foreign key that relates ParentCategoryID back up to CategoryID . When creating the FK, which would be the primary column, and which would be the foreign column in this instance?

Answer 1

I would think that ParentCategoryID would have to be NULL able (unless you want a top-level parent to point to itself, and that doesn't make much sense).

ALTER TABLE dbo.Categories
  ADD CONSTRAINT FK_SelfParent
  FOREIGN KEY (ParentCategoryID)
  REFERENCES dbo.Categories(CategoryID);

True story: at a job interview at Microsoft (several years ago), a SQL Server person told me that this wasn't possible.