联接替换子查询

Question

I am almost a novie in database queries. However,I do understand why and how correlated subqueries are expensive and best avoided. Given the following simple example - could someone help replacing with a join to help understand how it scores better:

SQL> select
  2    book_key,
  3    store_key,
  4    quantity
  5  from
  6    sales s
  7  where
  8    quantity < (select max(quantity)
  9                 from sales
 10                 where book_key = s.book_key); 

Apart from join,what other option do we have to avoid the subquery.

Answer 1

In this case, it ought to be better to use a windowed-function on a single access to the table - like so:

with s as
(select book_key, 
        store_key, 
        quantity, 
        max(quantity) over (partition by book_key) mq
 from sales)
select book_key, store_key, quantity
from s 
where quantity < s.mq

Answer 2

Using Common Table Expressions (CTE) will allow you to execute a single primary SELECT statement and store the result in a temporary result set. The data can then be self-referenced and accessed multiple times without requiring the initial SELECT statement to be executed again and won't require possibly expensive JOINs. This solution also uses ROW_NUMBER() and the OVER clause to number the matching BOOK_KEYs in descending order based off of the quantity. You will then only include the records that have a quantity that is less than the max quantity for each BOOK_KEY.

with CTE as
(
  select 
    book_key, 
    store_key, 
    quantity,
    row_number() over(partition by book_key order by quantity desc) rn
  from sales
)
select 
  book_key, 
  store_key, 
  quantity 
from CTE where rn > 1;

Working Demo: http://sqlfiddle.com/#!3/f0051/1

Answer 3

Apart from join,what other option do we have to avoid the subquery.

You use something like this:

SELECT select max(quantity)
INTO @myvar 
from sales
where book_key = s.book_key

select book_key,store_key,quantity
from sales s
where quantity < @myvar