Question

select model from (
    select price, model from pc where price = (select max(price) from pc)
    union
    select price, model from laptop where price = (select max(price) from laptop)
    union
    select price, model from printer where price = (select max(price) from printer)
) t1 where price = (select max(price) from (
    select price, model from pc where price = (select max(price) from pc)
    union
    select price, model from laptop where price = (select max(price) from laptop)
    union
    select price, model from printer where price = (select max(price) from printer)
) t2 )

I’m very new to SQL so my question is very simple, but I would like to sort out one point. Am I right that this query can not be simplified to something like this?

select model from (
    select price, model from pc where price = (select max(price) from pc)
    union
    select price, model from laptop where price = (select max(price) from laptop)
    union
    select price, model from printer where price = (select max(price) from printer)
) t1 where price = (select max(price) from t1)

And if it can not be, is it a bad thing that we run two same subqueries?

Answer 1

I still say to go with one table, which is best practice design. (Not duplicating identical tables unnecessarily.)

CREATE TABLE unified_table (
  product_type,
  price,
  model
)

Doing so enables this query…

SELECT
  *
FROM
  unified_table
WHERE
  price = (SELECT MAX(price) FROM unified_table)

But, if you can’t, or won’t, trust the optimiser to deal with the consequences of the UNIONs…

SELECT
  *
FROM
(
  SELECT * FROM pc
  UNION ALL
  SELECT * FROM laptop
  UNION ALL
  SELECT * FROM printer
) t1
WHERE
  price = (SELECT MAX(price) FROM (SELECT price FROM pc
                                   UNION ALL
                                   SELECT price FROM laptop
                                   UNION ALL
                                   SELECT price FROM printer
                                  ) t2
          )

The optimiser will understand how to optimise this so as to remove redundant searches.

EDIT:

As a compromise, you can make a unified view, and query that…

CREATE VIEW unified_table AS
  SELECT 'pc'      AS type, * FROM pc
  UNION ALL
  SELECT 'laptop'  AS type, * FROM laptop
  UNION ALL
  SELECT 'printer' AS type, * FROM printer