groupArrayArray

CREATE TABLE website_visits (
    user_id UInt32,
    session_id UInt32,
    page_visits Array(String)
) ENGINE = Memory;

INSERT INTO website_visits VALUES
(101, 1, ['homepage', 'products', 'checkout']),
(101, 2, ['search', 'product_details', 'contact']),
(102, 1, ['homepage', 'about_us']),
(101, 3, ['blog', 'homepage']),
(102, 2, ['products', 'product_details', 'add_to_cart', 'checkout']);

SELECT
    user_id,
    groupArrayArray(page_visits) AS user_session_page_sequences
FROM website_visits
GROUP BY user_id;

   ┌─user_id─┬─user_session_page_sequences───────────────────────────────────────────────────────────────┐
1. │     101 │ ['homepage','products','checkout','search','product_details','contact','blog','homepage'] │
2. │     102 │ ['homepage','about_us','products','product_details','add_to_cart','checkout']             │
   └─────────┴───────────────────────────────────────────────────────────────────────────────────────────┘