For certain reason, sometime we want to filter the data in a multi-values column.

e.g.

books

id isbn title
1 8-230185-1321 The Secret C++
2 23801-23815-9 MySQL for tummy
3 78-923722-223 Programmer Bible

users

id username name
1 js JS
2 foo Mr Foo
3 bar Ms Bar

user_books

book_id user_id date
2 1 2019-01-03 12:38:29
1 3 2019-01-08 18:08:09
3 2 2019-01-13 22:37:12

Let say now want to filter the books has borrowed by user ID 1 & 3

If normal select

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SELECT books.id AS book_id
  , books.isbn
  , books.title
  , t_borrowers.user_ids
FROM books
  LEFT JOIN user_books ON user_books.book_id = books.id
WHERE user_books.user_id IN (1, 3);

Filter in single column

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SELECT books.id AS book_id
  , books.isbn
  , books.title
  , t_borrowers.user_ids
FROM books
  LEFT JOIN (
    SELECT user_books.book_id
      , CONCAT('#', GROUP_CONCAT(user_books.user_id SEPARATOR '#,#'), '#') AS user_ids
    FROM user_books
    GROUP BY user_books.book_id
  ) AS t_borrowers ON t_borrowers.book_id = books.id
WHERE (t_borrowers.user_ids LIKE '%#1#%' OR t_borrowers.user_ids LIKE '%#3#%');

The trick is, to add a symbol (here I use #) to wrap the ID when concat the values,
then in bottom there use LIKE & OR to filter, remember to wrap around the ID when filter.

This example may not exactly shows up the purpose of using sub-query & wrap the value with symbol.
It look complicated & uncessary, but, in some cases may need to do in this way.