mysql> SELECT t1.ID,t1.THE_QUESTIONNAIRE_FK FROM questioncategory AS t1 LEFT JOIN questionnaire AS t2 ON ( t1.THE_QUESTIONNAIRE_FK = t2.ID) WHERE t1.THE_QUESTIONNAIRE_FK IS NOT NULL AND t2.ID IS NULL;
+-------------+----------------------+
| ID          | THE_QUESTIONNAIRE_FK |
+-------------+----------------------+
| 360_EN-Text | NULL                 |
+-------------+----------------------+
1 row in set (0.00 sec)

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)