Question

我试图避免子查询，因为它们通常比正确的连接具有更低的性能。

这是我目前无效的查询：

SELECT 
    a.`email_list_id`, a.`category_id`, a.`name`
FROM 
    `email_lists`AS a 
    LEFT JOIN `email_autoresponders` AS b 
        ON ( a.`website_id` = b.`website_id` ) 
WHERE 
    a.`website_id` = [...]
    AND a.`category_id` <> 0 
    AND a.`email_list_id` <> b.`email_list_id` 
GROUP BY 
    a.`email_list_id` 
ORDER BY a.`name`

此查询有效：

SELECT 
    `email_list_id`, `category_id`, `name` 
FROM 
    `email_lists` 
WHERE 
    `website_id` = [...] 
    AND `category_id` <> 0 
    AND `email_list_id` NOT IN ( 
        SELECT 
            `email_list_id` 
        FROM 
            `email_autoresponders` 
        WHERE `website_id` = [...] 
    ) 
GROUP BY 
    `email_list_id` 
ORDER BY 
    `name`

有没有办法用左连接做到这一点？我尝试了很多不同的选择。

Answer 1

重新思考一下之后，我相信这可能有用：

SELECT 
    a.`email_list_id`, a.`category_id`, a.`name`
FROM 
    `email_lists`AS a 
    LEFT JOIN `email_autoresponders` AS b 
        ON ( a.`website_id` = b.`website_id` and a.`email_list_id` = b.`email_list_id` ) 
WHERE 
    a.`website_id` = [...]
    AND a.`category_id` <> 0 
    AND b.`email_list_id` is NULL
GROUP BY 
    a.`email_list_id` 
ORDER BY a.`name`

Answer 2

对于初学者，将email_list_id上的不等式检查添加到连接条件，而不是将其放在WHERE子句中：

LEFT JOIN `email_autoresponders` AS b 
    ON ( a.`website_id` = b.`website_id` 
         AND a.`email_list_id` <> b.`email_list_id` )

虽然我不确定你提到的场景需要大量优化，但这是一种使用连接而不是子查询的方法......

MySQL - LEFT JOIN而不是IN

2 个答案: