MySQL Procedure/trigger/constraint - mysql

Question

I need help in creating a procedure with a constraint, when inserting staff member a manager to be specific he/she needs to be assigned to one clinic and one clinic only, thus when trying to insert a another manager to the same clinic it should create a trigger, that a manager has been assigned already to that clinic. Here's my code for inserting my variables :
CREATE DEFINER=`root`#`localhost` PROCEDURE `insert_staff`(in staff_num char(4),in First_name varchar(40),in surname varchar(30),in street varchar(20), in city varchar(20), in state varchar(2),in zipcode varchar(20),in Phone_no varchar(20),in Date_of_birth date,in gender_ char(1), in ID_no int(13),in Positon varchar(10),in salary_ double,in ClinicNo char(5))
BEGIN
set sql_safe_updates=0;
set foreign_key_checks=0;
insert into staff(staffNo,sFName,sLname,sStreet,sCity,sState,sZipCode,sTelNo,DOB,gender,ID,position,salary,clinicNo)
values(staff_num,First_name,surname,street,city,state,zipcode,Phone_no,Date_of_birth,gender_,ID_no,Positon,salary_,ClinicNo);
END

Answer 1

Following would check before inserting by selecting. If does not exist, record is inserted.
CREATE DEFINER=`root`#`localhost` PROCEDURE `insert_staff`(in staff_num char(4),in First_name varchar(40),in surname varchar(30),in street varchar(20), in city varchar(20), in state varchar(2),in zipcode varchar(20),in Phone_no varchar(20),in Date_of_birth date,in gender_ char(1), in ID_no int(13),in Positon varchar(10),in salary_ double,in ClinicNo char(5))
BEGIN
set sql_safe_updates=0;
set foreign_key_checks=0;
IF ( Position = 'Manager' AND EXISTS (select * from staff where clinicNo = ClinicNo and Positon='Manager')) THEN
//Handle your case
ELSE
insert into staff(staffNo,sFName,sLname,sStreet,sCity,sState,sZipCode,sTelNo,DOB,gender,ID,position,salary,clinicNo)values(staff_num,First_name,surname,street,city,state,zipcode,Phone_no,Date_of_birth,gender_,ID_no,Positon,salary_,ClinicNo);
END IF;
END