2個聯合查詢之間的區別

以下2個查詢有什麼區別？（它們都產生相同的結果）2個聯合查詢之間的區別

select * 
from (
    select * from (
      select * 
      from phppos_items 
      where name like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16 
      ) t 
    union 
    select * from (
      select * 
      from phppos_items 
      where item_number like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16 
      ) t 
    union 
    select * from (
      select * 
      from phppos_items 
      where category like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16 
      ) t 
) as top_rows 
order by `name` limit 16

select * 
from (
      (select * 
      from phppos_items 
      where name like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16) 
    union 
      (select * 
      from phppos_items 
      where item_number like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16) 
    union 
      (select * 
      from phppos_items 
      where category like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16) 
) as top_rows 
order by `name` limit 16

來源

2011-05-30 Chris Muench

第一個版本還有另一組臨時表，這是無用的，資源在這種特殊情況下的浪費。
以下所有會產生相同的結果：

SELECT * FROM T1; 

SELECT * FROM (SELECT * FROM T1); 


SELECT * FROM (SELECT * FROM (SELECT * FROM T1)); 

... 
...

到無窮遠:-)

並在您的非常具體的情況下，這將是不夠的：

select * 
      from phppos_items 
      where 
       (name like 'AB10LA2%' OR item_number like 'AB10LA2%' OR category like 'AB10LA2%') 
      and deleted = 0 
      order by `name` limit 16

來源

2011-05-30 16:50:31

臨時表？我很確定兩個查詢都會產生相同的執行計劃。 – Magnus 2011-05-30 16:56:24

原件不應該優先考慮按名稱找到的物品。 – 2011-05-30 17:01:37

@Jonathan Leffler你是對的，編輯 – 2011-05-30 17:04:21

第一個查詢嵌入了一個額外的一套SELECT * FROM (sub-query)。優化器需要稍微長一些的時間來整理它。

在功能上，查詢之間沒有真正的區別。第二個更簡單，因此更可取。

來源

2011-05-30 16:50:50

我看到的唯一區別是在頂部，UNION的子查詢組件被命名。顯然，由於第二個版本沒有錯誤，所以MySQL在這個構造中不需要子查詢名稱。順便說一句，你有沒有測試過這個版本比使用OR的相應單個查詢快？

來源

2011-05-30 16:52:56

2個聯合查詢之間的區別

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