查询多个或vs联合多个查询，哪一个会更快？

Question

顾名思义，与多个或将查询更快或查询与多个查询加入联盟会更快？我只加入了两个表格，使其变得简单，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

我怎么没看到你的工会的会更快。您必须扫描并加入3次，与第一次一样，您只扫描一次，对于每个加入的记录，您都可以应用这些条件。

Answer 2

由于您只查询一次数据，因此效率最高的一个会更有效率。也就是说，如果您查询的是不会返回的数据，则最好将这些条件放入WHERE的IN子句中。在您提供的例子，它是这样的：

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%') 

随着过滤柱正确的索引（PUBLISHERNAME，在这种情况下），MySQL将优化每个部件和更有效地将它们结合起来。

Answer 3

第二个会更快。

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