Question

Why is the following foreign key constraint (although executes fine) not enforced by SQLite? How can I go about enforcing the relationship?

CREATE TABLE User (
    UserID TEXT Unique NOT NULL PRIMARY KEY,
    FirstName TEXT NOT NULL,
    LastName TEXT NOT NULL,
    Username TEXT NOT NULL,
    Password TEXT NOT NULL,
    Email TEXT NOT NULL,
    SignupDate TEXT NOT NULL
)

CREATE TABLE Category (
    CategoryID TEXT Unique NOT NULL PRIMARY KEY,
    UserID TEXT,
    FOREIGN KEY(UserID) REFERENCES User(UserID)
)

Answer 1

As the relevant docs say (in section 2. Enabling Foreign Key Support):

Assuming the library is compiled with
foreign key constraints enabled, it
must still be enabled by the
application at runtime, using the
PRAGMA foreign_keys command. For
example:

sqlite> PRAGMA foreign_keys = ON;

Foreign key constraints are disabled
by default (for backwards
compatibility), so must be enabled
separately for each database
connection separately.

Have you used that PRAGMA in the relevant connection? (Assuming, as the docs say, that sqlite is compiled appropriately, and also a recent-enough version to offer foreign key constraint enforcement, of course).