The WITH clause
The UNION of three tables yields multiple rows of the same product_id with different prices and supplier names, and some with no item info. Can we generate a query to return a distinct set of records based on product_id with the cheapest price and the name of the supplier with that cheapest price?
The UNION of three tables yields multiple rows of the same product_id with different prices and supplier names, and some with no item info:
product_id supplier price iteminfo widget Fred 100 info1 widget Joe 90 NULL widget Billy 95 info3
Can we generate a query to return a distinct set of records based on product_id with the cheapest price and the name of the supplier with that cheapest price? And/or any one of the filled info fields? Or do we need to step up to PL/SQL? Thanks in advance.
If your data were in a simple table, the query you would use involves a correlated subquery:
select product_id
, supplier
, price
, iteminfo
from simpletable as T
where price =
( select min(price)
from simpletable
where product_id = T.product_id )
The only thing we have to change to fit your specific circumstances is to replace simpletable with a derived table consisting of the result of your UNION:
select product_id
, supplier
, price
, iteminfo
from (
select ... from table1
union all
select ... from table2
union all
select ... from table3
) as T
where price =
( select min(price)
from (
select ... from table1
union all
select ... from table2
union all
select ... from table3
) as T1
where product_id = T.product_id )
Note that we had to make this substitution twice. This sure is clumsy, and yes, there is a better way to write this, using the WITH clause:
with simpletable as
(
select ... from table1
union all
select ... from table2
union all
select ... from table3
)
select product_id
, supplier
, price
, iteminfo
from simpletable as T
where price =
( select min(price)
from simpletable
where product_id = T.product_id )
This way of "abstracting" the derived table using the WITH clause makes the SQL manageable again. Note that the WITH clause is SQL-99, so not every database system supports it. In Oracle, you need to be on at least Oracle 9i release 2.
