mysql - 使用 WHERE 子查询改进 SQL

Question

我的 SQL 运行良好，但有点慢。理想情况下，希望在 where 子句中有额外的子查询，类似于 AND country_iso = "uk"。

SELECT c.content_name, c.content_telephone, c.content_address1
    (SELECT w.weblink_url FROM weblinks w WHERE w.content_id = c.content_id ORDER BY w.weblink_ordering, w.weblink_id ASC LIMIT 1) 
    AS content_url,                                                          
     (SELECT country_name FROM countries WHERE country_id = c.country_id LIMIT 1)
    AS country_name,                                                                                                                     
     (SELECT country_iso FROM countries WHERE country_id = c.country_id LIMIT 1)
    AS country_iso
    FROM catcontents cc                                                     
    LEFT JOIN content c ON c.content_id = cc.content_id                                                         
    WHERE cc.category_id = 7 
    AND (SELECT country_iso FROM countries WHERE country_id = c.country_id LIMIT 1) = 'uk' 
ORDER BY cc.catcontent_ordering ASC, c.content_name ASC

Answer 1

此查询是否不符合您的需求？ (基本上，使用连接来获取国家，而不是子选择。)

SELECT
    c.content_name,
    c.content_telephone,
    c.content_address1,

    countries.country_name,
    countries.country_iso,

    (
        SELECT w.weblink_url 
        FROM weblinks w 
        WHERE w.content_id = c.content_id 
        ORDER BY w.weblink_ordering, w.weblink_id ASC 
        LIMIT 1
    ) content_url

FROM catcontents cc
LEFT JOIN content c ON c.content_id = cc.content_id
JOIN countries ON countries.country_id = c.country_id

WHERE cc.category_id = 7
AND countries.country_iso = 'uk'

ORDER BY cc.catcontent_ordering ASC, c.content_name ASC