Update table inside a loop in a procedure pl/sql - sql

Question

So i want to hire a new manager for a department. This procedure has 2 parameters, the department name i want to change, and the new id for manager (taken from employee's id). So to change it, i need to update all employee that has the old manager id, and change their manager id into the new one. This is my code so far, the problem is that it updated all of the employees' manager, so the whole database is updated. I must use procedure, not function. Any idea? Thanks.
CREATE OR REPLACE PROCEDURE update_manager(v_deptname IN departments.department_name%TYPE,v_empid IN employees.employee_id%TYPE) IS
v_deptid departments.department_id%type;
BEGIN
SELECT department_id INTO v_deptid FROM departments WHERE department_name=v_deptname;
FOR i IN (SELECT * FROM employees)
LOOP
IF i.department_id=v_deptid THEN
UPDATE employees
SET manager_id=v_empid
WHERE i.department_id=v_deptid;
END IF;
END LOOP;
END;
/
BEGIN
update_manager('Marketing',100);
END;
/

Answer 1

If you must create a PL/SQL procedure then you can do
CREATE OR REPLACE PROCEDURE update_manager(v_deptname IN departments.department_name%TYPE,v_empid IN employees.employee_id%TYPE) IS
v_deptid departments.department_id%type;
BEGIN
SELECT department_id INTO v_deptid FROM departments WHERE department_name=v_deptname;
UPDATE employees
SET manager_id=v_empid
WHERE department_id=v_deptid;
END;
/
The problem with your code that's causing "it updated all of the employees' manager" is that your update statement:
UPDATE employees
SET manager_id=v_empid
WHERE i.department_id=v_deptid;
Your filter here is comparing i.department_id, this is the variable that's coming from your FOR i IN (SELECT * FROM employees), NOT from the update statement. You've already confirmed that i.department_id=v_deptid because you are calling this in a loop with an if statement checking it.
It is not efficient at all to get all rows in employees, loop the results, checking each row if it matches a condition and then firing off an update statement (even if your update statement is filtering against the correct row.

Answer 2

I don't have your tables, but I do have Scott's so - here's an example.
Departments and employees in department 10:
SQL> select * From dept order by deptno;
DEPTNO DNAME LOC
---------- -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL> select deptno, empno, ename, mgr
2 from emp
3 where deptno = 10;
DEPTNO EMPNO ENAME MGR
---------- ---------- ---------- ----------
10 7782 CLARK 7839
10 7839 KING
10 7934 MILLER 7782
Procedure that looks like yours (as you're studying PL/SQL), using a variable to fetch department number into, as well as a loop:
SQL> create or replace procedure update_manager
2 (v_deptname in dept.dname%type,
3 v_empno in emp.empno%type
4 )
5 is
6 v_deptno dept.deptno%type;
7 begin
8 select d.deptno
9 into v_deptno
10 from dept d
11 where d.dname = v_deptname; --> this is what you are missing
12
13 for cur_r in (select e.empno
14 from emp e
15 where e.deptno = v_deptno
16 )
17 loop
18 update emp a set
19 a.mgr = v_empno
20 where a.empno = cur_r.empno;
21 end loop;
22 end;
23 /
Procedure created.
Testing: let's modify manager of all employees in department 10:
SQL> exec update_manager ('ACCOUNTING', 7839);
PL/SQL procedure successfully completed.
SQL> select deptno, empno, ename, mgr
2 from emp
3 where deptno = 10;
DEPTNO EMPNO ENAME MGR
---------- ---------- ---------- ----------
10 7782 CLARK 7839
10 7839 KING 7839
10 7934 MILLER 7839
SQL>
Seems to work. Though, that's inefficient; doing it row-by-row (in a loop) is usually slow-by-slow. Work on the whole sets of data, rather. Something like this:
SQL> rollback;
Rollback complete.
SQL> create or replace procedure update_manager
2 (v_deptname in dept.dname%type,
3 v_empno in emp.empno%type
4 )
5 is
6 begin
7 update emp e set
8 e.mgr = v_empno
9 where e.deptno = (select d.deptno
10 from dept d
11 where d.dname = v_deptname
12 );
13 end;
14 /
Procedure created.
SQL> exec update_manager ('ACCOUNTING', 7782);
PL/SQL procedure successfully completed.
SQL> select deptno, empno, ename, mgr
2 from emp
3 where deptno = 10;
DEPTNO EMPNO ENAME MGR
---------- ---------- ---------- ----------
10 7782 CLARK 7782
10 7839 KING 7782
10 7934 MILLER 7782
SQL>

Answer 3

Why not a simple select for manager id and an update on all concerned employee ?
DECLARE #department_id uniqueidentifier
SELECT #department_id = departement_id
FROM departments
WHERE department_name = #v_deptname
UPDATE employees
SET manager_id = #v_empid
WHERE department_id = #department_id