SQL表連接的另一個表

Question

我有2個表（maintable和categorytable）

maintable記錄：

categoryCode field2 field3 

1    XXXXXX1 ACTIVE 
3    XXXXXX2 ACTIVE 
1    XXXXXX3 ACTIVE 
1    XXXXXX4 ACTIVE 
3    XXXXXX5 ACTIVE 
3    XXXXXX5 NOT ACTIVE 

categorytable記錄：

categoryCode  categoryname 

1     categoryname1 
2     categoryname2 
3     categoryname3 

到目前爲止，我有這個疑問

SELECT COUNT(*) AS recordcount, 
     categoryCode AS catCode, 
     categorytable.categoryname 
    FROM maintable, 
     categorytable 
    WHERE categorytable.categoryCode = maintable.categoryCode 
    AND maintable.field3 = 'ACTIVE' 
GROUP BY maintable.categoryCode 
ORDER BY categorytable.categoryCode 

with下面的輸出

recordcount catCode categoryname 
---------------------------------- 
3   1  categoryname1 
2   3  categoryname3 

但我需要的輸出是這樣的（0記錄類別中包括顯示爲0的RecordCount）：

recordcount catCode categoryname 
3   1  categoryname1 
0   2  categoryname2 
2   3  categoryname3 

Answer 1

您需要使用左連接：

SELECT COUNT(*) AS recordcount, categoryCode AS catCode, categorytable.categoryname 
FROM categorytable 
LEFT JOIN maintable ON categorytable.categoryCode = maintable.categoryCode 
GROUP BY maintable.categoryCode 
ORDER BY categorytable.categoryCode 

如果你不想扭轉表名，請參閱AlexanderMP的答案。

Answer 2

SELECT 
    coalesce(COUNT(maintable.categoryCode),0) AS recordcount, 
    categorytable.categoryCode AS catCode, 
    categorytable.categoryname 
FROM 
    maintable 
RIGHT JOIN categorytable ON categorytable.categoryCode = maintable.categoryCode 
GROUP BY 
    categorytable.categoryCode, 
    categorytable.categoryname 
ORDER BY 
    categorytable.categoryCode 

，或者用左連接

SELECT 
    coalesce(COUNT(maintable.categoryCode),0) AS recordcount, 
    categorytable.categoryCode AS catCode, 
    categorytable.categoryname 
FROM 
    categorytable 
LEFT JOIN maintable ON categorytable.categoryCode = maintable.categoryCode 
GROUP BY 
    categorytable.categoryCode, 
    categorytable.categoryname 
ORDER BY 
    categorytable.categoryCode