Question

我有访问级别和页面。页数＆lt; = many-to-many =＆gt;水平我需要计算每个级别有多少打开的页面。但如果pageCount为0，则不会出现这种情况。

SELECT 
    levels.id,
    levels.name,
    COUNT(pages.id) as pageCount
FROM levels
LEFT JOIN page_levels
    ON levels.id = page_levels.level_id
LEFT JOIN wp_pages
    ON page_levels.page_id = pages.id
WHERE pages.status = 'open'
GROUP BY levels.id

令人沮丧。请告诉我我错过了什么。

Answer 1

将条件移至ON（）。

SELECT 
    levels.id,
    levels.name,
    COUNT(pages.id) as pageCount
FROM levels
LEFT JOIN page_levels
    ON levels.id = page_levels.level_id
LEFT JOIN wp_pages AS pages
    ON page_levels.page_id = pages.id AND pages.status = 'open'
GROUP BY levels.id

如果计数0，则不返回Mysql结果

1 个答案: