mySQL JOIN不會返回結果0計數

Question

SELECT categories.*, COUNT(categoryID) AS kritCount 
FROM categories AS categories 
LEFT JOIN krits ON categories.id = categoryID 
WHERE (krits.approved = '1') 
GROUP BY categories.id 

所以這個工程很好，除了它不返回類別中有0個krits的類別。

它將如果我刪除了WHERE語句，但我需要的WHERE，只選擇其中場批准= 1

Answer 1

的krits試試這個：

SELECT categories. * , COUNT(categoryID) AS kritCount FROM categories AS categories 
LEFT JOIN krits ON categories.id = categoryID 
WHERE (krits.approved = '1' OR krits.approved IS NULL) 
GROUP BY categories.id 

Answer 2

任何時候你從引用列在where子句中留下連接的表（除了測試NULL值），則強制該連接的行爲類似於內部連接。相反，將您的測試移出where子句並使其成爲連接條件的一部分。

SELECT categories. * , COUNT(categoryID) AS kritCount 
    FROM categories AS categories 
     LEFT JOIN krits 
      ON categories.id = categoryID 
       AND krits.approved = '1' 
    GROUP BY categories.id 

Answer 3

從閱讀查詢，它看起來像你想打電話類別，每個類別，您希望爲各個類別Krits（批准）的計數，並且當有沒有，你還願意類別，但顯示爲0的個性化......

試試這個

select 
     cat.*, 
     COALESCE(kc.KritCount, 0) as KritCount 
    from 
     Categories cat 
     left join (select k.CategoryID, 
          count(*) KritCount 
         from 
          Krits k 
         where 
          k.approved = '1' 
         group by 
          k.CategoryID) kc 
      on cat.id = kc.CategoryID