Adding or alter foreign key to a table in mysql

Question

I am new beginner on sql query on mysql. I have table structure like this :

---------------------------------        ----------------------------
|  Table office                 |        |  Table_detail_complaint  |
---------------------------------        ----------------------------
|(PK)  id_office                |    ----|(PK) id_complaint         |
|      name                     |    |   |     complaint_2          |
|      complaint_1              |    |   |     time_response_2      |
|      time_response_complaint1 |    |   |     complaint_3          |
|      address                  |    |   |     time_response_3      |
|(FK)  id_complaint             |-----   ----------------------------
---------------------------------

I got a problem like this.

1. I have success to create the table office. But, I forgot to put id_complaint as a foreign key. How can I ?
2. How to create the detail_complaint_table ?
3. After adding the fk, how can I make like this, if " a row in table office is deleted also delete a row on table_detail_complaint". Using cascade or what ?
4. How to query complaint_1, complaint_2, complaint_3 on a select query, using JOIN or what ?
5. This is the my big problem. I have a case that if the complaint is continuesly until complaint_N ?. Should I make complaint_4, complaint_5 ? Or, have another idea ?

Answer 1

You can create foreign key and cascading for child auto delete as per below-

ALTER TABLE table_office 
ADD CONSTRAINT FK_table_office_id_complaint FOREIGN KEY (id_complaint) 
REFERENCES table_detail_complaint (id_complaint) 
ON DELETE CASCADE;

For other part of your question share more details what kind of information you want to store and how you will use them..