Question

此作品

SELECT DISTINCT  b.b_id 
FROM             b INNER JOIN c 
ON               b.b_id=c.b_id  
WHERE            c.active='yes' AND b.featured='no'

当结果返回0行时，返回一个count = 0的空行

SELECT DISTINCT  b.b_id, COUNT(c.c_id) AS count
FROM             b INNER JOIN c 
ON               b.b_id=c.b_id  
WHERE            c.active='yes' AND b.featured='no'

我做错了吗？

Answer 1

我认为您需要left join而不是inner join，因为当给定{没有匹配的c记录时，您希望返回0而不是丢失的行数{1}}记录。

此外，在使用聚合函数时，您应该包含b，例如group by。

count

Answer 2

尝试将DISTINCT 放入 COUNT()：

SELECT b.b_id, COUNT(DISTINCT c.c_id) AS count
FROM b
JOIN c ON b.b_id=c.b_id  
WHERE c.active='yes' 
AND b.featured='no'
GROUP BY 1

顺便说一句，请考虑像这样格式化你的SQL，这是更可读的恕我直言。

Answer 3

我想尝试GROUP BY而不是DISTINCT。

    SELECT b.b_id, COUNT(c.c_id) AS count
    FROM 
    b INNER JOIN c 
    ON 
    b.b_id=c.b_id  
    WHERE 
    c.active='yes' 
    AND 
    b.featured='no'
    GROUP BY b.b_id

MySQL - 使用COUNT选择返回NULL行

3 个答案: