Query with many sub-selects on the same table

Question

Hi,
Please consider the example below (Oracle 11g):
create TABLE test1 (order_id number, CODE VARCHAR2(10), VALUE NUMBER);
insert into test1 VALUES (1, 'A', 100);
insert into test1 VALUES (1, 'B', 200);
insert into test1 VALUES (1, 'C', 300);
insert into test1 VALUES (1, 'D', 400);
insert into test1 VALUES (2, 'A', 10);
insert into test1 VALUES (2, 'B', 20);
SELECT order_id,
       CODE,
       VALUE,
       (SELECT VALUE FROM test1 t1
       WHERE t1.order_id = t.order_id
       AND   t1.code = 'B') b_Value,
       (SELECT SUM(VALUE)/COUNT(*) FROM test1 t1
       WHERE t1.order_id = t.order_id
       AND   t1.code IN ('C', 'D')) cd_Value,
       (SELECT COUNT(*) FROM test1 t1
       WHERE t1.order_id = t.order_id
       AND   t1.code IN ('C', 'D')) cd_qty
FROM   TEST1 t
WHERE  CODE = 'A';In my real-life case, I don't have 3 but tens of sub-query columns like those. The performance is OK but I was wondering whether there were a better way in terms of performance and maintainability to write this query.
I thought of the WITH clause but couldn't see how it could help much.
Please note that some sub-queries can be more complex than just looking up a column, like in column cd_value/cd_qty.
Any suggestions?
Thanks
Luis

Answer 1

Simpler, but will go for two table scans. You will have to use outer joins according to your requirement.
select t1.order_id,
       t1.CODE,
       t1.VALUE,
       max(decode(t2.code,'B',t2.value)) b_value,
       sum(decode(t2.code,'C',t2.value,'D',t2.value,0))/nullif(sum(decode(t2.code,'C',1,'D',1,0)),0)  cd_value,
       sum(decode(t2.code,'C',1,'D',1,0))  cd_qty 
from TEST1 t1,test1 t2
where t1.CODE = 'A'
and t1.ORDER_ID = t2.order_id
group by t1.order_id,
       t1.CODE,
       t1.VALUEEdited by: jeneesh on Feb 7, 2012 11:54 AM