按非空值分组

Question

我有一个数据库：

+--+----+---------+
|id|name|source_id|
+--+----+---------+
|11|aaaa|null
+--+----+---------+
|12|bbbb|1
+--+----+---------+
|13|cccc|1
+--+----+---------+
|14|dddd|null
+--+----+---------+
|15|eeee|2
+--+----+---------+
|16|ffff|2
+--+----+---------+
|17|gggg|2
+--+----+---------+

我想选择所有条目，但有一个例外：

• 与的所有条目source_id == somevalue(not null)均应分组
• *与的 *最后一个条目source_id == null应在之后

这是我期望的结果：

+----+--+----+---------+
COUNT|id|name|source_id|
+----+--+----+---------+
.....|11|aaaa|null
+----+--+----+---------+
.2...|13|cccc|1
+----+--+----+---------+
.....|14|dddd|null
+----+--+----+---------+
.3...|17|ffff|2
+----+--+----+---------+

到目前为止，我已经做到了这一点（注意：“ change_count” = COUNT）：

SELECT *, (To_days(date_expires)-TO_DAYS(NOW())) as dayDiff, COUNT(id) AS change_count FROM mytable
GROUP BY source_id 
HAVING dayDiff < 4 
ORDER BY date_created DESC

那就是我得到的：

+----+--+----+---------+
COUNT|id|name|source_id|
+----+--+----+---------+
.2...|11|aaaa|null
+----+--+----+---------+
.2...|12|bbbb|1
+----+--+----+---------+
.3...|15|eeee|2
+----+--+----+---------+

如您所见，结果有两个问题：

• 条目按首次source_id出现分组（应为last（最新））
• nulls 也被分组（不应该被分组）

我上面描述的任务可以实现吗？我该怎么做？

编辑：

SELECT *, COUNT(id) AS change_count FROM 
(SELECT *, (To_days(date_return_due)-TO_DAYS(NOW())) as dayDiff FROM mytable 
WHERE owner_id='1' 
HAVING dayDiff < 100 
ORDER BY date_created DESC) AS newtable
GROUP BY (CASE WHEN source_id IS NULL THEN id ELSE source_id END)
ORDER BY (CASE WHEN source_id IS NULL THEN 1 ELSE 0 END), date_created DESC";

Answer 1

我认为以下是您想要的：

SELECT *, (To_days(date_expires)-TO_DAYS(NOW())) as dayDiff, COUNT(id) AS change_count
FROM mytable
GROUP BY (case when source_id is null then id else source_id end)
HAVING dayDiff < 4 
ORDER BY (case when source_id is null then 1 else 0 end), date_created DESC

它是有条件的，group by因此NULL sourceids将不会被分组。然后，使用中的逻辑将它们置于最后order by。

我不明白您最后一次发生的意思。现在我想我做了：

SELECT coalesce(s.id, mytable.id) as id,
       max(case when s.maxid is not null and s.maxid = myable.id then mytable.name
                when s.maxid is null then NULL
                else mytable.name
           end) as name,
       (To_days(date_expires)-TO_DAYS(NOW())) as dayDiff, COUNT(id) AS change_count
FROM mytable left outer join
     (html" target="_blank">select source_id, MAX(id) as maxid
      from mytable
      where source_id is not null
      group by source_id
     ) s
     on mytable.id = s.maxid
GROUP BY (case when source_id is null then id else source_id end)
HAVING dayDiff < 4 
ORDER BY (case when source_id is null then 1 else 0 end), date_created DESC

这将加入最新记录中的信息（基于最高ID）。