Short-circuit logic evaluation operators

Question

Are there any short-circuit logic operators (specifically short-circuit AND
and short-circuit OR) that I can use in a WHERE clause in MySQL 5.5? If
there isn’t, what are the alternatives?

An abstract view at my problem along with an explanation as to why I need this
can be found at this fiddle:

http://sqlfiddle.com/#!2/97fd1/3

In reality we are looking at millions of books in millions of bookstores in
thousands of cities in hundreds of countries, which is why we cannot accept
the overhead of receiving the unneeded information with every query we
dispatch and seriously need to find a way to make the evaluation stop as soon
as we have all rows that satisfy the current condition, before moving on to
the next OR.

Let me know if you need more information. Thanks in advance.

根据要求，以下是小提琴中使用的架构：

CREATE TABLE quantitycache (
  id INT AUTO_INCREMENT,
  quantity INT,
  book_id INT NOT NULL,
  bookstore_id INT NULL,
  city_id INT NULL,
  country_id INT NULL,
  PRIMARY KEY (id)
);

以及一些示例数据：

INSERT INTO quantitycache 
     (quantity, book_id, bookstore_id, city_id, country_id)
VALUES
     (5,        1,       1,            NULL,    NULL),
     (100,      2,       1,            NULL,    NULL),
     (7,        1,       2,            NULL,    NULL),
     (12,       1,       NULL,         1,       NULL),
     (12,       1,       NULL,         NULL,    1),
     (100,      2,       NULL,         1,       NULL),
     (100,      2,       NULL,         NULL,    1),
     (200,      3,       NULL,         1,       NULL),
     (250,      3,       NULL,         NULL,    1);

Answer 1

请记住，查询不是强制执行的。您编写的查询可能在多个线程上运行，因此where子句中的短路运算符不会仅导致一个结果。

而是，使用LIMIT子句仅返回第一行。

SELECT * FROM quantitycache
WHERE bookstore_id = 1 OR city_id = 1 OR country_id = 1
ORDER BY bookstore_id IS NULL ASC,
         city_id IS NULL ASC,
         country_id IS NULL ASC
LIMIT 1;

要获得结果集中所有书籍的最佳匹配，请将结果保存到临时表中，找到最佳结果，然后返回有趣的字段。

CREATE TEMPORARY TABLE results (id int, book_id int, match_rank int);

INSERT INTO results (id, book_id, match_rank)
SELECT id, book_id, 
    -- this assumes that lower numbers are better
    CASE WHEN Bookstore_ID is not null then 1 
         WHEN City_ID is not null then 2 
         ELSE 3 END as match_rank
FROM quantitycache
WHERE bookstore_id = 1 OR city_id = 1 OR country_id = 1;

Select * 
from (
    select book_id, MIN(match_rank) as best_rank 
    from results 
    group by book_id
) as r
inner join results as rid 
    on r.book_id = rid.book_id 
    and rid.match_rank = r.best_rank
inner join quantitycache as q on q.id = rid.id;

DROP TABLE results;