表連接和分組數目查詢

查詢

查詢此表中library_id=6的以topic_id和topic_name爲一組的數量

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

關於兩個表的連接查詢

表一

表二

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讓表一的module_id和表二的id連接，並使表一的module_id,topic_id,topic_name和表二的name爲一組的數量

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;

結果如下