Is there a way to reuse subqueries in the same query? - teradata

Question

See Update at end of question for solution thanks to marked answer!
I'd like to treat a subquery as if it were an actual table that can be reused in the same query. Here's the setup SQL:
create table mydb.mytable
(
id integer not null,
fieldvalue varchar(100),
ts timestamp(6) not null
)
unique primary index (id, ts)
insert into mydb.mytable(0,'hello',current_timestamp - interval '1' minute);
insert into mydb.mytable(0,'hello',current_timestamp - interval '2' minute);
insert into mydb.mytable(0,'hello there',current_timestamp - interval '3' minute);
insert into mydb.mytable(0,'hello there, sir',current_timestamp - interval '4' minute);
insert into mydb.mytable(0,'hello there, sir',current_timestamp - interval '5' minute);
insert into mydb.mytable(0,'hello there, sir. how are you?',current_timestamp - interval '6' minute);
insert into mydb.mytable(1,'what up',current_timestamp - interval '1' minute);
insert into mydb.mytable(1,'what up',current_timestamp - interval '2' minute);
insert into mydb.mytable(1,'what up, mr man?',current_timestamp - interval '3' minute);
insert into mydb.mytable(1,'what up, duder?',current_timestamp - interval '4' minute);
insert into mydb.mytable(1,'what up, duder?',current_timestamp - interval '5' minute);
insert into mydb.mytable(1,'what up, duder?',current_timestamp - interval '6' minute);
What I want to do is return only rows where FieldValue differs from the previous row. This SQL does just that:
locking row for access
select id, fieldvalue, ts from
(
--locking row for access
select
id, fieldvalue,
min(fieldvalue) over
(
partition by id
order by ts, fieldvalue rows
between 1 preceding and 1 preceding
) fieldvalue2,
ts
from mydb.mytable
) x
where
hashrow(fieldvalue) <> hashrow(fieldvalue2)
order by id, ts desc
It returns:
+----+---------------------------------+----------------------------+
| id | fieldvalue | ts |
+----+---------------------------------+----------------------------+
| 0 | hello | 2015-05-06 10:13:34.160000 |
| 0 | hello there | 2015-05-06 10:12:34.350000 |
| 0 | hello there, sir | 2015-05-06 10:10:34.750000 |
| 0 | hello there, sir. how are you? | 2015-05-06 10:09:34.970000 |
| 1 | what up | 2015-05-06 10:13:35.470000 |
| 1 | what up, mr man? | 2015-05-06 10:12:35.690000 |
| 1 | what up, duder? | 2015-05-06 10:09:36.240000 |
+----+---------------------------------+----------------------------+
The next step is to return only the last row per ID. If I were to use this SQL to write the previous SELECT to a table...
create table mydb.reusetest as (above sql) with data;
...I could then do this do get the last row per ID:
locking row for access
select t1.* from mydb.reusetest t1,
(
select id, max(ts) ts from mydb.reusetest
group by id
) t2
where
t2.id = t1.id and
t2.ts = t1.ts
order by t1.id
It would return this:
+----+------------+----------------------------+
| id | fieldvalue | ts |
+----+------------+----------------------------+
| 0 | hello | 2015-05-06 10:13:34.160000 |
| 1 | what up | 2015-05-06 10:13:35.470000 |
+----+------------+----------------------------+
If I could reuse the subquery in my initial SELECT, I could achieve the same results. I could copy/paste the entire query SQL into another subquery to create a derived table, but this would just mean I'd need to change the SQL in two places if I ever needed to modify it.
Update
Thanks to Kristján, I was able to implement the WITH clause into my SQL like this for perfect results:
locking row for access
with items (id, fieldvalue, ts) as
(
select id, fieldvalue, ts from
(
select
id, fieldvalue,
min(fieldvalue) over
(
partition by id
order by ts, fieldvalue
rows between 1 preceding and 1 preceding
) fieldvalue2,
ts
from mydb.mytable
) x
where
hashrow(fieldvalue) <> hashrow(fieldvalue2)
)
select t1.* from items t1,
(
select id, max(ts) ts from items
group by id
) t2
where
t2.id = t1.id and
t2.ts = t1.ts
order by t1.id

Answer 1

Does WITH help? That lets you define a result set you can use multiple times in the SELECT.
From their example:
WITH orderable_items (product_id, quantity) AS
( SELECT stocked.product_id, stocked.quantity
FROM stocked, product
WHERE stocked.product_id = product.product_id
AND product.on_hand > 5
)
SELECT product_id, quantity
FROM orderable_items
WHERE quantity < 10;