-- Sub-course prerequisite links (admin-defined course flow)
CREATE TABLE IF NOT EXISTS sub_course_prerequisites (
    id BIGSERIAL PRIMARY KEY,
    sub_course_id BIGINT NOT NULL REFERENCES sub_courses(id) ON DELETE CASCADE,
    prerequisite_sub_course_id BIGINT NOT NULL REFERENCES sub_courses(id) ON DELETE CASCADE,
    created_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,

    UNIQUE(sub_course_id, prerequisite_sub_course_id),
    CHECK(sub_course_id != prerequisite_sub_course_id)
);

CREATE INDEX IF NOT EXISTS idx_sub_course_prerequisites_sub_course_id
    ON sub_course_prerequisites(sub_course_id);
CREATE INDEX IF NOT EXISTS idx_sub_course_prerequisites_prerequisite_id
    ON sub_course_prerequisites(prerequisite_sub_course_id);

-- User progress per sub-course
CREATE TABLE IF NOT EXISTS user_sub_course_progress (
    id BIGSERIAL PRIMARY KEY,
    user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    sub_course_id BIGINT NOT NULL REFERENCES sub_courses(id) ON DELETE CASCADE,
    status VARCHAR(20) NOT NULL DEFAULT 'IN_PROGRESS' CHECK (
        status IN ('IN_PROGRESS', 'COMPLETED')
    ),
    progress_percentage SMALLINT NOT NULL DEFAULT 0 CHECK (
        progress_percentage BETWEEN 0 AND 100
    ),
    started_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
    completed_at TIMESTAMPTZ,
    created_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMPTZ,

    UNIQUE(user_id, sub_course_id)
);

CREATE INDEX IF NOT EXISTS idx_user_sub_course_progress_user_id
    ON user_sub_course_progress(user_id);
CREATE INDEX IF NOT EXISTS idx_user_sub_course_progress_sub_course_id
    ON user_sub_course_progress(sub_course_id);
CREATE INDEX IF NOT EXISTS idx_user_sub_course_progress_status
    ON user_sub_course_progress(user_id, status);