查詢多個或vs聯合多個查詢，哪一個會更快？

Question

顧名思義，與多個或將查詢更快或查詢與多個查詢加入聯盟會更快？我只加入了兩個表格，使其變得簡單，但可以有更多的表格加入。

例如，

select isbn, booktitle from book as b left join publisher as p 
     on b.publisherid = p.publisherid 
     where 
      booktitle like '%mysql%' or 
      isbn like '%mysql%' or 
      publishername like '%mysql%' 

VS

 select isbn, booktitle from book as b left join publisher as p 
    on b.publisherid = p.publisherid 
    where 
    booktitle like '%mysql%' 
    union 
    select isbn, booktitle from book as b left join publisher as p 
    on b.publisherid = p.publisherid 
    where 
    isbn like '%mysql%' 
    union 
    select isbn, booktitle from book as b left join publisher as p 
    on b.publisherid = p.publisherid 
    where 
    publishername like '%mysql%' 

Answer 1

我怎麼沒看到你的工會的會更快。您必須掃描並加入3次，與第一次一樣，您只掃描一次，對於每個加入的記錄，您都可以應用這些條件。

Answer 2

由於您只查詢一次數據，因此效率最高的一個會更有效率。也就是說，如果您查詢的是不會返回的數據，則最好將這些條件放入WHERE的IN子句中。在您提供的例子，它是這樣的：

SELECT isbn, booktitle 
FROM book as b 
WHERE 
    booktitle LIKE '%mysql%' or 
    isbn like '%mysql%' or 
    b.publisherid in (SELECT publisherid FROM publisher WHERE publishername like '%mysql%') 

隨着過濾柱正確的索引（PUBLISHERNAME，在這種情況下），MySQL將優化每個部件和更有效地將它們結合起來。

Answer 3

第二個會更快。

下面是分析：

http://books.google.com.sg/books?id=iaCCQ13_zMIC&pg=PT118&lpg=PT118&dq=In+this+example,+MySQL+uses+the+ExpireTime+index+to+fetch+a+set+of+rows&source=bl&ots=3FJ1-70Hq3&sig=V_0z7nSV7_hGwW_fC8IWmFI7ZLw&hl=en&sa=X&ei=Xx1uUan5IsulrQfh04GIBA&ved=0CDQQ6AEwAA