Question

类别表

+----+------+
| id | name |
+----+------+
|  1 | cat1 |
|  2 | cat2 |
|  3 | cat3 |
+----+------+

产品表

+----+-------+--------+   
| id | name  | cat_id |    
+----+-------+--------+
|  1 | prod1 |      1 |    
|  2 | prod2 |      1 |    
|  3 | prod3 |      3 |    
|  4 | prod4 |      1 |    
|  5 | prod5 |      3 |
|  6 | prod6 |      1 |    
+----+-------+--------+

这是我的疑问：

 SELECT c.id, c.name, count( p.cat_id ) num
    FROM categories c
    INNER JOIN products p ON c.id = p.cat_id
    GROUP BY p.cat_id

结果是：

+----+------+-------+    
| id | name | count |    
+----+------+-------+    
|  1 | cat1 |     4 |    
|  3 | cat3 |     2 |  
+----+------+-------+

现在出现错误显示计数cat = 2 如何修复得到这个结果：

+----+------+-------+    
| id | name | count |    
+----+------+-------+    
|  1 | cat1 |     4 |    
|  2 | cat2 |     0 |    
|  3 | cat3 |     2 |  
+----+------+-------+

此处的完整示例：http://sqlfiddle.com/#!9/f2cbb9/1

Answer 1

我认为，您只需将联接更改为左联接。

SELECT c.id, c.name, count( p.cat_id ) num
        FROM categories c
        LEFT JOIN products p ON c.id = p.cat_id
        GROUP BY p.cat_id

Answer 2

您需要使用left join

 SELECT c.id, c.name, count( p.cat_id ) num
    FROM categories c
    LEFT JOIN products p ON c.id = p.cat_id
    GROUP BY p.cat_id

参考：What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?

Answer 3

SELECT c.id, c.name, count( p.cat_id ) num
        FROM categories c
        LEFT JOIN products p ON c.id = p.cat_id
        GROUP BY p.cat_id

Answer 4

每个答案都是正确的，您需要使用LEFT JOIN来达到指定的结果。要获得您请求的完全结果，您需要执行以下操作：

SELECT c.id, c.name, COUNT(p.cat_id) AS `count`
FROM categories AS c
LEFT JOIN products AS p ON c.id = p.cat_id
GROUP BY p.cat_id
ORDER BY c.id ASC

SO question Boby referenced很好地解释了为什么你不能使用INNER JOIN来达到理想的结果。

查询按类别计算产品时出错

4 个答案: