子句中的子选择是执行多次还是执行一次？

Question

I have a separate table (with no PK, only one row) to store the row count of a certain result. Now, this count is used in a query, like this (just an example):

SELECT * FROM `car`
WHERE `car`.`featured` = 1
AND ( SELECT `count` FROM `carfeaturedcounter` ) > 5;

Will SELECT count FROM carfeaturedcounter be executed multiple times (once for every row), or only once per query?

Answer 1

I think it depends on the version of MySQL. You might as well move the condition to the from clause, where it will only be executed once:

SELECT c.*
FROM car c.CROSS JOIN
     (SELECT `count` FROM carfeaturedcounter) x
WHERE c.featured = 1 AND x.`count` > 5;