Sql query to be solved the salary of employee dealing with customer having id=4 has been doubled

create table employee
	(emp_id 	integer	 	not null,
	 manager_id 	integer		not null,	
	 emp_name 	char(20)	not null,
	 emp_tel 	char(10),
  	 emp_salary	number		not null,
	 hire_date	date,
	constraint pk_employee primary key(emp_id)
	);

create table customer
	(cust_id	integer 	not null,
	cust_name 	char(20)	not null ,
	cust_address 	varchar2(200)	,
	emp_id 		integer	 	not null,
    	constraint pk_customer primary key(cust_id)
	);

alter table customer 
add constraint fk_employee_customer foreign key(emp_id) 
references employee(emp_id);

alter table employee 
add constraint fk_employee_manager foreign key(manager_id) 
references employee(emp_id);

The salary of employee dealing with customer having id=4 has been doubled

update employee
set emp_salary=emp_salary*2
from employee e
join customer c
on c.emp_id=e.emp_id
where e.emp_id =(4)