mysql> select THE_QUESTIONNAIRE_FK from questioncategory where ID='360_EN-Text';
+----------------------+
| THE_QUESTIONNAIRE_FK |
+----------------------+
| NULL                 |
+----------------------+
1 row in set (0.00 sec)

mysql> select THE_QUESTIONNAIRE_FK from questioncategory where ID='360_EN-Text' AND THE_QUESTIONNAIRE_FK IS NULL;
Empty set (0.00 sec)

mysql> select LENGTH(THE_QUESTIONNAIRE_FK) from questioncategory where ID='360_EN-Text';
+------------------------------+
| LENGTH(THE_QUESTIONNAIRE_FK) |
+------------------------------+
|                            0 |
+------------------------------+
1 row in set (0.00 sec)