Un poco complicado, pero podría crear objetos JSON para cada fila, concatenarlos usando GROUP_CONCAT y emitir el resultado (envuelto en [] para convertirlo en una matriz) a JSON;
SELECT u.uid, u.username,
CASE WHEN p.uid IS NULL
THEN NULL
ELSE CAST(CONCAT('[',
GROUP_CONCAT(JSON_OBJECT('pid', p.pid,
'title', p.title,
'status', p.status)),
']') AS JSON) END portfolios
FROM user u
LEFT JOIN portfolio p
ON u.uid=p.uid
WHERE p.status = 1
GROUP BY u.uid, u.username;
...que da...
+------+----------+-----------------------------------------------------------------------------------------------------------------------------------------+
| 1 | manoj | [{"pid": 1, "title": "title 1", "status": 1}, {"pid": 2, "title": "title 2", "status": 1}, {"pid": 3, "title": "title 3", "status": 1}] |
| 2 | testing | [{"pid": 4, "title": "title 1", "status": 1}] |
| 3 | user | NULL |
| 4 | company | NULL |
+------+----------+-----------------------------------------------------------------------------------------------------------------------------------------+
Si está utilizando un MySQL anterior sin compatibilidad con JSON, puede compilarlo como una cadena;
SELECT u.uid, u.username,
CASE WHEN p.uid IS NULL
THEN NULL
ELSE CONCAT('[',
GROUP_CONCAT(CONCAT('{ "pid":',p.pid,',"title":"', REPLACE(p.title, '"', '\\"'),
'","status":',p.status, ' }')), ']') END portfolios
FROM user u
LEFT JOIN portfolio p
ON u.uid=p.uid AND p.status=1
GROUP BY u.uid, u.username;
...que te dará...
+------+----------+------------------------------------------------------------------------------------------------------------------------------+
| uid | username | portfolios |
+------+----------+------------------------------------------------------------------------------------------------------------------------------+
| 1 | manoj | [{ "pid":2,"title":"title 2","status":1 },{ "pid":3,"title":"title 3","status":1 },{ "pid":1,"title":"title 1","status":1 }] |
| 2 | testing | [{ "pid":4,"title":"title 1","status":1 }] |
| 3 | user | NULL |
| 4 | company | NULL |
+------+----------+------------------------------------------------------------------------------------------------------------------------------+
