表连接和分组数目查询

查询

查询此表中library_id=6的以topic_id和topic_name为一组的数量

SELECT topic_id AS topicid,topic_name AS topicname,COUNT(*) AS num FROM `comment` WHERE library_id = 6 GROUP BY topic_id,topic_name; 

关于两个表的连接查询

表一

表二

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让表一的module_id和表二的id连接，并使表一的module_id,topic_id,topic_name和表二的name为一组的数量

SELECT
    b.name,
    a.module_id AS moduleid,
    a.topic_id AS topicid,
    a.topic_name AS topicname,
    COUNT(*) AS num
FROM
    `comment` a
LEFT JOIN application_tree b ON a.module_id = b.id
WHERE
    a.library_id = 6
GROUP BY
    a.topic_id,
    a.topic_name,
    a.module_id,
    b. name;

结果如下