STRING_AGG / GROUP_CONCAT

-- PostgreSQL / SQL Server (string_agg)
SELECT user_id,
       STRING_AGG(name, ', ' ORDER BY name) AS tag_list
FROM   user_tags
GROUP  BY user_id;

-- MySQL (group_concat) — same idea, slightly different syntax
SELECT user_id,
       GROUP_CONCAT(name ORDER BY name SEPARATOR ', ') AS tag_list
FROM   user_tags
GROUP  BY user_id;

-- DISTINCT + custom separator + sort
SELECT order_id,
       STRING_AGG(DISTINCT product_name, ' | ' ORDER BY product_name)
FROM   order_items
GROUP  BY order_id;

-- ⚠️ MySQL truncates at group_concat_max_len (default 1024).
-- For long lists:  SET SESSION group_concat_max_len = 1000000;