Dfyjkt

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!

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.

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.