mysql - 忽略 COUNT 中的重复行

Question

以下查询计算了 exp_judging 表中有多少行，字段 pre 中的值为 1，其中 exp_submissions 中还有一个匹配列，其 member_group 为 5

SELECT COUNT(*) AS count_result
  FROM exp_judging AS jud
  LEFT JOIN exp_submissions AS sub ON jud.rel_id = sub.id 
  WHERE jud.pre = 1
        AND sub.member_group = 5

这很好用。但是，将来在 'exp_judging' 表中会有多个具有相同 'rel_id' 的行。

如何确保每行只计算一个？

我尝试将 GROUP BY 添加到末尾，但它只显示 1 个结果，而不是之前的 4 个。（还没有重复）

GROUP BY jud.rel_id

编辑：

尝试了以下方法，但仍然只显示 1：

SELECT COUNT(DISTINCT jud.rel_id) AS count_result
FROM exp_judging AS jud
LEFT JOIN exp_submissions AS sub ON jud.rel_id = sub.id 
WHERE jud.pre = 1
AND sub.member_group = $member_group
GROUP BY jud.rel_id

Answer 1

使用COUNT(DISTINCT rel_id)而不是COUNT(*).

Answer 2

试试下面的查询

 SELECT COUNT(jud.rel_id) AS count_result
 FROM exp_judging AS jud
 LEFT JOIN exp_submissions AS sub ON jud.rel_id = sub.id 
 WHERE jud.pre = 1
    AND sub.member_group = 5
group by jud.rel_id