Więc odpowiadam sobie. To, czego szukałem, to CONCAT_WS . Moja prośba staje się wtedy
SELECT t1.*,
GROUP_CONCAT(DISTINCT CONCAT_WS(
' ', t2.info, t2.data
) SEPARATOR ', ') AS info,
GROUP_CONCAT(DISTINCT t3.email SEPARATOR '|') AS email
FROM table1 AS t1
LEFT JOIN link1 AS l1
ON l1.id_a = t1.id_a
LEFT JOIN table2 AS t2
ON t2.id_b = l1.id_b
LEFT JOIN link2 AS l2
ON l2.id_a = t1.id_a
LEFT JOIN table3 AS t3
ON t3.id_c = l2.id_c
WHERE t1.id_a = ?
GROUP BY t1.id_a
