我们如何从表中获取多计数值？

Question

我有一个包含以下列的数据库表：'dialogue', 'user_id',和'readed'。我想为每次对话计算未读信息。

例如：

select count(messageid) from messages where user_id = $id and readed=0 and diyalogid = $diyalog1

select count(messageid) from messages where user_id = $id and readed=0 and diyalogid = $diyalog2

select count(messageid) from messages where user_id = $id and readed=0 and diyalogid = $diyalog3

如何只用一个查询计算计数？

Answer 1

你可以这样做：

SELECT
    SUM(CASE WHEN diyalogid = $diyalog1 THEN 1 ELSE 0 END) AS diyalog1Count,
    SUM(CASE WHEN diyalogid = $diyalog2 THEN 1 ELSE 0 END) AS diyalog2Count,
    SUM(CASE WHEN diyalogid = $diyalog3 THEN 1 ELSE 0 END) AS diyalog3Count
FROM 
    messages 
WHERE
    user_id = $id 
    AND readed=0

修改

回答你的评论。我不太确定你想要什么。你可以这样做：

SELECT
    COUNT(*) AS theCount,
    diyalogid
FROM 
    messages 
WHERE
    user_id = $id 
    AND readed=0
GROUP BY
    diyalogid 

然后您将按diyalogid

计算