What are the better ways with INSERT on relational tables?

[supernoob]

I'm experiencing my first mysql INSERT tests and I'm using these tables:\[code\]table employees-> employee_id-> employee_name-> employee_surname-> employee_urltable areas-> area_id-> area_city-> area_address-> area_countrytable agencies-> agency_id-> agency_name-> agency_urltable node_employees-> node_id-> employee_id-> area_id-> agency_id\[/code\]I would store data in table_employee, table_areas and table_agency but I'm not forced to save all the data simultaneously, so I could create an employee, and subsequently an agency or an address.In a case of singular data insert, should I use something like this or shoud I use directly the table node_employees, if yes, how can I do it?\[code\]INSERT INTO employees (employee_name, employee_surname, employee_url)VALUES ('Roger', 'Waters', 'http://pinkfloyd.com')INSERT INTO agencies (agency_name, agency_url)VALUES ('Google', 'http://google.com')INSERT INTO areas (area_city, area_address, area_country)VALUES ('Rome', 'Via Roma, 123', 'Italy')\[/code\]To link rows each other I've created node_employees, a relational table.I use it to link an employee with an area or an agency, so what I should do to link data with this relational table?\[code\]SELECT employee_id FROM employees WHERE employee_name = 'Roger'SELECT agency_id FROM agencies WHERE agency_name = 'Google'// I'll get their ids in php$php_employee_id$php_agency_id// and thenINSERT INTO node_employees (employee_id, agency_id)VALUES ('$php_employee_id', '$php_agency_id')\[/code\]I have also another doubt, what I should do if I need to link an employee with an area? shoud I use a different query, so a query for every possibility?\[code\]// so, not this$php_employee_id = 12;$php_agency_id = 7;$php_area_id = null;INSERT INTO node_employees (employee_id, agency_id, area_id)VALUES ('$php_employee_id', '$php_agency_id', '$php_area_id') // will this remove the previous data with null in area_id?\[/code\]