SQL查詢簡化

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) 

我對SQL很陌生，所以我的問題很簡單，但我想解決一點。我是對的，這個查詢不能被簡化成這樣的東西？

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) 

如果這是不可能的，是什麼讓我們運行兩個相同的子查詢壞事？

Answer 1

我還是說要去一張桌子，這是最好的實踐設計。 （不是重複相同的表不必要的。）

CREATE TABLE unified_table (
    product_type, 
    price, 
    model 
) 

這樣做使此查詢...

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

但是，如果你不能或不會，相信優化器來處理與聯盟的後果...

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 
     ) 

優化程序將瞭解如何優化，以消除多餘的搜索。

編輯：

作爲妥協，你可以做一個統一的視圖，查詢...

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

嘗試這樣：

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 

不過我建議你檢查你的數據庫結構，這似乎是你創建了多個表基於類型相同的東西（的項目）。您可以將所有這些保留在一個表中，僅通過類型列的內容進行區分。

無極限：

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 

既然你有3個值進行比較，從不同的表，沒有關係，你必須做一個聯盟，然後比較它們

這是一種方法，你不需要再次計算價格。