忽略COUNT中的重复行

Question

以下查询给出exp_judging表中有多少行的计数，字段pre中的值为1，其中exp_submissions中的匹配列的成员组为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

这很有效。但是，将来在'exp_judging'表中会有多行具有相同的'rel_id'。

如何确保每行只计算一行？

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

GROUP BY jud.rel_id

编辑：

尝试过以下内容，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)代替COUNT(*)。

Answer 2

尝试以下查询

 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