Invalid reference to variable PL/SQL - oracle

Question

I'm having trouble in getting started with PL/SQL
Here is my code:
SET SERVEROUTPUT ON;
DECLARE
v_cname customers.customer_name%type := '&customer_name';
v_cardno customers.card_number%type := '&card_number';
v_lastcid customers.customer_id%type;
BEGIN
SELECT customer_id INTO v_lastcid from customers
where customer_id = (select max(customer_id) from customers);
dbms_output.put_line(v_lastcid);
INSERT INTO customers(customer_id, customer_name, card_number)
VALUES(v_lastcid.NEXTVAL, v_cname, v_cardno);
COMMIT;
END;
This returns an error:
ORA-06550: line 12, column 20:
PLS-00487: Invalid reference to variable 'V_LASTCID'
ORA-06550: line 12, column 20:
PL/SQL: ORA-02289: sequence does not exist
ORA-06550: line 11, column 13:
PL/SQL: SQL Statement ignored
06550. 00000 - "line %s, column %s:\n%s"
*Cause: Usually a PL/SQL compilation error.
Any help would me much appreciated!

Answer 1

You probably meant to say MAX + 1 (see lines 7 and 12), i.e.
SQL> create table customers
2 (customer_id number,
3 customer_name varchar2(20),
4 card_number varchar2(20));
Table created.
SQL> DECLARE
2 v_cname customers.customer_name%type := '&customer_name';
3 v_cardno customers.card_number%type := '&card_number';
4 v_lastcid customers.customer_id%type;
5
6 BEGIN
7 SELECT nvl(max(customer_id), 0) INTO v_lastcid from customers
8 where customer_id = (select max(customer_id) from customers);
9 dbms_output.put_line(v_lastcid);
10
11 INSERT INTO customers(customer_id, customer_name, card_number)
12 VALUES(v_lastcid + 1, v_cname, v_cardno);
13 COMMIT;
14 END;
15 /
Enter value for customer_name: Little
Enter value for card_number: Foot
0
PL/SQL procedure successfully completed.
SQL> select * from customers;
CUSTOMER_ID CUSTOMER_NAME CARD_NUMBER
----------- -------------------- --------------------
1 Little Foot
SQL>
Although it works, it is doomed to fail in a multi-user environment if two (or more) users fetch the same MAX value; insert would fail with a DUP-VAL-ON-INDEX error (if the ID is supposed to be unique).
Therefore, use a sequence (which is what NEXTVAL in your code suggests):
SQL> create sequence seq_cust;
Sequence created.
SQL> DECLARE
2 v_cname customers.customer_name%type := '&customer_name';
3 v_cardno customers.card_number%type := '&card_number';
4 v_lastcid customers.customer_id%type;
5 BEGIN
6 INSERT INTO customers(customer_id, customer_name, card_number)
7 VALUES(seq_cust.nextval, v_cname, v_cardno);
8 COMMIT;
9 END;
10 /
Enter value for customer_name: Big
Enter value for card_number: Foot
PL/SQL procedure successfully completed.

Answer 2

nextval is used to get the next value from a sequence. It's usually used to create a pseudo primary key. You can think of it like a special function that can only be called on sequences.
v_lastcid is a variable that has the same type as the customer_id column in the customers table. So if you had a table like this...
CREATE TABLE CUSTOMERS ( CUSTOMER_ID INTEGER );
...then v_lastcid is an integer;
If you're trying to make a dummy customer with the next highest number maybe you mean something like...
INSERT INTO customers(customer_id, customer_name, card_number)
VALUES(v_lastcid + 1, v_cname, v_cardno);
Hope this helps.