sql - 向已存在的子查询引入简单计数子查询

Question

我正在创建一个连接到SQLite数据库的简单图像浏览器。在浏览器中，相似的图像被分组为一个事件，并且每个图像都带有一些标签。

有人帮助我在下面构造了这个非常有用的查询。它包含5个随机标签作为示例：

SELECT * 
FROM 
    (SELECT 
         t.EventId,
         SUM(CASE WHEN name = 'necktie' THEN 1 ELSE 0 END) as 'necktie',
         SUM(CASE WHEN name = 'shirt' THEN 1 ELSE 0 END) as 'shirt',
         SUM(CASE WHEN name = 'suit' THEN 1 ELSE 0 END) as 'suit',
         SUM(CASE WHEN name = 'man' THEN 1 ELSE 0 END) as 'man',
         SUM(CASE WHEN name = 'male' THEN 1 ELSE 0 END) as 'male' 
     FROM 
         TagsMSCV t 
     WHERE 
         name IN ('necktie', 'shirt', 'suit', 'man', 'male') 
     GROUP BY 
         t.EventId) 
ORDER BY 
    COUNT(*) DESC

这将返回每个事件中显示的每个标签（列称为“名称”）的数量。但是，现在我还需要事件的大小（事件中唯一图像ID的数量），可以通过以下查询来完成：

SELECT EventId, COUNT(DISTINCT ImageId) 
FROM TagsMSCV 
GROUP BY EventId 

但是我不知道如何在上面的子查询中引入这种语法？如果我将其放在 t.EventId旁边，则仅在使用不正确的5个随机标记标记的情况下才对图像ID进行计数。我需要事件中的总唯一图像ID。

Answer 1

首先，您的子查询不是必需的。其次，您可以使用条件COUNT(DISTINCT)：

SELECT t.EventId,
        SUM(CASE WHEN name = 'necktie' THEN 1 ELSE 0 END) as necktie,
        SUM(CASE WHEN name = 'shirt' THEN 1 ELSE 0 END) as shirt,
        SUM(CASE WHEN name = 'suit' THEN 1 ELSE 0 END) as suit,
        SUM(CASE WHEN name = 'man' THEN 1 ELSE 0 END) as man,
        SUM(CASE WHEN name = 'male' THEN 1 ELSE 0 END) as male 
        COUNT(DISTINCT CASE WHEN name = 'necktie' THEN imageid END) as necktie_images,
        COUNT(DISTINCT CASE WHEN name = 'shirt' THEN imageid END) as shirt_images,
        COUNT(DISTINCT CASE WHEN name = 'suit' THEN imageid END) as suit_images,
        COUNT(DISTINCT CASE WHEN name = 'man' THEN imageid END) as man_images,
        COUNT(DISTINCT CASE WHEN name = 'male' THEN imageid END) as male_images
FROM TagsMSCV t
WHERE name IN ('necktie', 'shirt', 'suit', 'man', 'male')
GROUP BY t.EventId 

编辑：

如果您想要一个事件的全部不同图像，则不需要条件逻辑。只需使用：

COUNT(DISTINCT imageid) as total_images,

并删除 WHERE子句。