MySQL 查询在同一个表上有多个内部连接

Question

我有两个表： tblBooks 和 tblBookMeta

tblBookMeta 保存书籍的类型，并且可能不止一种。

使用此查询，我得到所有结果，如图所示。

SELECT * FROM tblBooks as B inner join tblBookMeta as BM ON B.BookId = BM.BookId 

tblBooks                           tblBookMeta
BookId  Title                   |  BookMetaId   BookId  MetaName    MetaValue   
1       My Thriller Book        |  1            1       Genre       Thriller    
2       My SciFi Book           |  2            2       Genre       SciFi   
3       My Thriller SciFi Book  |  3            3       Genre       Thriller    
3       My Thriller SciFi Book  |  4            3       Genre       SciFi   

要按类型查询书籍，我可以使用此查询

SELECT * FROM tblBooks as B
inner join tblBookMeta as BM ON B.BookId = BM.BookId 
WHERE MetaName = 'Genre' and MetaValue = 'Thriller'

这将返回“My Thriller Book”和“My Thriller SciFi book”。 美好的

如果我只想要同时具有惊悚类型和科幻类型的书籍，我会有点卡住。 我可以这样做，但我想知道是否有没有子查询的更好方法。 我将来可能想要查询两个以上的类型，这将创建另一个子查询。

SELECT * FROM tblBooks as B
inner join tblBookMeta as BM ON B.BookId = BM.BookId 
WHERE MetaName = 'Genre' and MetaValue = 'Thriller' 
and B.BookId in (SELECT B.BookId FROM tblBooks as B
    inner join tblBookMeta as BM ON B.BookId = BM.BookId 
    WHERE MetaName = 'Genre' and MetaValue = 'SciFi') 

Answer 1

您可以执行类似的操作并检查 EXPLAIN ANALYZE 更好的解决方案。

SELECT 
    *
FROM 
  tblBooks as B inner join tblBookMeta as TH
    ON (B.BookId = TH.BookId and TH.MetaValue = 'Thriller')
  inner join tblBookMeta as SC
    ON (B.BookId = SC.BookId and SC.MetaValue = 'SciFi')
WHERE 
    MetaName = 'Genre'

问候