title: Find intersection between two columns consisting of comma-separated items in BigQuery SQL
body: <p>I have several columns in a joined table which we can call 'sessions_and_transactions' with data on web sessions transacting (purchasing one or several items) on an ecommerce platform. Two of these columns are 'products_added_to_cart_from_product_page' and 'products_purchased' which are both of string data type and consisting of comma-separated product id:s. Commonly, the comma-separated products in 'products_added_to_cart_from_product_page' will also be in the 'products_purchased' column. Sometimes, however, the user doesn't purchase all or even any of the products added to cart product page. The user can also add products to cart from other pages than the product page (for instance, from a recommendations page).</p>
  <p>For each session (session_id), I would like to insert into a new column of all product_id:s in 'products_added_to_cart_from_product_page', that are also in 'products_purchased'. So basically, the set intersection of products.</p>
  <p>An example:
  <a href="https://i.stack.imgur.com/VZ5K2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VZ5K2.png" alt="enter image description here" /></a></p>
  <p>I remind you that the comma-separated lists of items are not of array data type</p>