How to assign a value to variable from select statement (PL/SQL Developer) - oracle

Question

I'm working with PL/SQL Developer.
I'm trying to update values in a column(existing table).
The values used to populate rows should be auto incremented. The starting value is the maximum value that already exist in such field.
An example, I have the following table
ORDER_ID T_NAME T_PRICE
20 CAR 50
NULL VAN 100
NULL BIKE 10
NULL BOAT 300
After running the query I would expect the table to look like:
ORDER_ID T_NAME T_PRICE
20 CAR 50
21 VAN 100
22 BIKE 10
23 BOAT 300
The query I created so far is:
DECLARE
temp_order_id number;
BEGIN
:temp_order_id = SELECT ISNULL(MAX((ORDER_ID)),0) + 1 FROM SALES_ACC;
update SALES_ACC
set (ORDER_ID) = :temp_order_id , :temp_order_id = :temp_order_id + 1
where (ORDER_ID) is null;
END;
Oracle doesn't like assigning a value from select statement to the temp_order_id variable.
Does anyone has any idea how to fix it?

Answer 1

You don't need pl/sql for this - you can do it in a single update statement - eg:
create table test1 as
select 20 order_id, 'CAR' t_name, 50 t_price from dual union all
select null order_id, 'VAN' t_name, 100 t_price from dual union all
select null order_id, 'BIKE' t_name, 10 t_price from dual union all
select null order_id, 'BOAT' t_name, 300 t_price from dual;
update test1
set order_id = (select max(order_id) from test1) + rownum
where order_id is null;
commit;
select * from test1
order by 1;
ORDER_ID T_NAME T_PRICE
---------- ------ ----------
20 CAR 50
21 VAN 100
22 BIKE 10
23 BOAT 300
drop table test1;
As a side note, it sounds like order_id is something that should really be the primary key of the table - if you had that, then you wouldn't be allowed to add a row without a value. Plus, you would also need a sequence that you would then use when inserting data into the table - e.g.:
insert into test1 (order_id, t_name, t_price)
values (test1_seq.nextval, 'TRIKE', 30);

Answer 2

ORACLE's recomended way for this is either:
Create a sequence and a trigger on the table to assign order_id as soon as row is being inserted
or, for Oracle 12c, you can have an IDENTITY column
See How to create id with AUTO_INCREMENT on Oracle?, both approaches are described there.

Answer 3

Within the DECLARE ... BEGIN ... END; section you are in PL/SQL syntax. That is not equal to the SQL syntax. Within PL/SQL syntax you should make use of the so called select into statement.
SELECT ISNULL(MAX((ORDER_ID)),0) + 1
into :temp_order_id
FROM SALES_ACC