PL/SQL procedure successfully completed, but when I want to display Max(DEPARTMENT_ID), can anyone pls suggest? - plsql

Question

Max(DEPARTMENT_ID), can anyone Please suggest
<set serveroutput on;
DECLARE
v_max_deptno DEPARTMENTS.DEPARTMENT_ID%TYPE;
BEGIN
SELECT Max(department_id)
INTO v_max_deptno
FROM departments
Where department_id = v_max_deptno;
DBMS_OUTPUT.PUT_LINE('The maximum department_id is: '|| v_max_deptno);
END;/>

Answer 1

This declaration:
v_max_deptno DEPARTMENTS.DEPARTMENT_ID%TYPE;
is for a local variable. Since you provide no default value and it is never assigned any value, it defaults to NULL.
This predicate in the SQL query:
Where department_id = v_max_deptno
means that the query should only return rows where the department_id is equal to the value in that variable. Since the variable is NULL, and since no value can equal NULL, the query returns no rows.
Since your query is based on a simple aggregate (MAX), the query returns without error. Since there were no rows, the MAX function returns NULL back into the same variable again.
For further assistance, you need to explain what the purpose of your code is and what value you expect to get. If you need the maximum department_id currently in the table across all rows, you would simply remove the where clause.
Note: if your purpose in finding the current maximum ID is to set the department_id for a new record, there are some other issues with this approach (to do with concurrent sessions) that you need to consider.

Answer 2

Your query doesn't make much sense, as it is trying to find the MAX(department_id) (and put its value into the V_MAX_DEPTNO variable), while the same variable is used in the WHERE clause.
At the time of declaration, V_MAX_DEPTNO is NULL, it is NULL in the WHERE clause as well, and no department's ID is "equal" to NULL, so that query returns nothing.
If you initialized the V_MAX_DEPTNO (which is, of course, stupid), you'd get something:
SQL> DECLARE
2 v_max_deptno departments.department_id%TYPE := 20;
3 BEGIN
4 SELECT MAX (department_id)
5 INTO v_max_deptno
6 FROM departments
7 WHERE department_id = v_max_deptno;
8
9 DBMS_OUTPUT.put_line ('The maximum department_id is: ' || v_max_deptno);
10 END;
11 /
The maximum department_id is: 20
PL/SQL procedure successfully completed.
SQL>
So the question is: what are you trying to do? In my opinion, the right way to do it is to remove this WHERE clause, i.e.
SQL> DECLARE
2 v_max_deptno departments.department_id%TYPE := 20;
3 BEGIN
4 SELECT MAX (department_id)
5 INTO v_max_deptno
6 FROM departments;
7
8 DBMS_OUTPUT.put_line ('The maximum department_id is: ' || v_max_deptno);
9 END;
10 /
The maximum department_id is: 40
PL/SQL procedure successfully completed.
SQL>