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 )

我对SQL很新，所以我的问题非常简单，但我想澄清一点。我是对的，这个查询不能简化为这样的东西吗？

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)

如果它不可能，我们运行两个相同的子查询是不是一件坏事？

Answer 1

我仍然说要用一张桌子，这是最佳实践设计。 （不会不必要地复制相同的表格。）

CREATE TABLE unified_table (
  product_type,
  price,
  model
)

这样做可启用此查询...

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

但是，如果你不能或不愿意相信优化者来处理联盟的后果......

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
          )

优化工具将了解如何对其进行优化，以便删除多余的搜索。

修改的

作为妥协，您可以制作统一的视图，并查询...

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

Answer 2

尝试这样的事情：

select model, price
from (
    select price, model from pc order by price desc limit 1
    union
    select price, model from laptop order by price desc limit 1
    union
    select price, model from printer order by price desc limit 1
) t1 
order by price desc
limit 1

但是我建议你检查一下你的数据库结构，这似乎就是你根据类型为同一个东西（项目）创建了多个表。您可以将所有这些保留在一个表中，仅通过类型列的内容进行区分。

无限制：

select t1.model, t1.price
from 
(select max(price) p
 from
    select max(price) p from pc
    union
    select max(price) p from laptop
    union
    select max(price) p from printer
) max_price
JOIN (
    select price, model from pc
    union
    select price, model from laptop
    union
    select price, model from printer
) t1 ON price >= max_price.p

Answer 3

select * 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)
) order by Price desc limit 1

由于你有3个值要比较，从不同的表中，没有关系，你必须做一个联盟，然后比较它们

这是一种方式，您无需再次计算价格。

SQL查询简化

3 个答案: