Question

我可以合并这两个SQL语句吗？目前正在运行2个查询试着收紧一点。

第一个：

SELECT * FROM (`cars`) 
JOIN `brands` ON `brands`.`br_id` = `cars`.`brand_id`
WHERE `cars`.`id` = '185707'

第二个：

SELECT ROUND(AVG(rating)) as avg_rating
FROM car_ratings WHERE car_id = 185707

Answer 1

您可以使用group by：

执行此操作

select cars.*,
       brands.*, 
       round(avg(car_ratings.rating)) as avg_rating
from   (cars
    inner join brands on brands.br_id = cars.brand_id)
    left join car_ratings on car_ratings.car_id = cars.id
where  cars.id = 185707
group by cars.id

请注意this is a MySQL extension to standard SQL;在标准SQL中，您需要列出group by子句中的所有选定字段。

Answer 2

select *

, (select round(avg(rating)) from car_ratings 
where car_id = cars.id) as avg_rating

from cars join brands on brands.br_id = cars.brand_id
where cars.id = 185707

但是，这是否代表了改进是另一个问题，最好通过查看正在使用的查询计划来回答。

MySQL - 我可以组合这两个SQL语句吗？结合JOIN和AVERAGE？

2 个答案: