php - 获取没有文件的文件夹

Question

我使用此查询来获取文件夹包含的文档数:

$query = " 
        SELECT
            *,
            count(*) as number_documents
        FROM
            table_documents
        LEFT JOIN
            table_folders ON folder_id = document_folder_id
        GROUP BY
            document_folder_id
        ORDER BY
            number_documents DESC
";

我无法获取包含 0 个文档的文件夹，如何让文件夹包含:

文件夹 1 - 100 个文件

文件夹 2 - 35 个文件

文件夹 3 - 0 个文件

表格:

**table_folders**
folder_id
folder_description
folder_active

**table_documents**
document_id
document_folder_id
document_description
document_file

Answer 1

这会起作用:

        SELECT
            *,
            sum(if (document_folder_id is null,0,1)) as number_documents
        FROM
            table_folders
        LEFT JOIN
            table_documents ON document_folder_id = folder_id
        GROUP BY
            folder_id
        ORDER BY
            number_documents DESC