Question

Im having problem trying to join 3 tables, every time I run the query on my server I just get a huge pause which requires manual termination.

here is the table strictures

books   : isbn(PK)
books_a : id(PK), isbn, price, condition
books_l : id(PK), isbn, price, condition

here is the query

SELECT 
   b.isbn, 
   a.price       AS a_price, 
   a.condition   AS a_condition, 
   l.price       AS l_price, 
   l.condition   AS l_condition 

FROM 
   books b  

   LEFT JOIN a_books a ON b.isbn  = a.isbn 
   LEFT JOIN l_books l ON b.isbn  = l.isbn

What could be the problem with my query? (note I do have over 6,000 records in each table)

Answer 1

I bet that one or some of these columns are not indexed. a_books.isbn, books.isbn, l_books.isbn

try running this statements,

ALTER table a_books ADD INDEX IDX_abk (isbn);
ALTER table books ADD INDEX IDX_bk (isbn);
ALTER table l_books ADD INDEX IDX_lbk (isbn);

Read this article: How does database indexing work?