Question

让我们假设有一个表 assignments ，其列为

assignment_id
分配人
task_id
受让人

相同的task_id可以由分配者分配给1个以上的受让人。目前，我有一个分配给2名女传教员的任务。让我们在下表中查看我的示例：

+---------------+----------+---------+----------+
| assignment_id | assignor | task_id | assignee | 
+---------------+--------------------+----------+
|       1       |    a1    |    t1   |    x1    | 
+---------------+----------+---------+----------+
|       2       |    a1    |    t1   |    x2    | 
+---------------+----------+---------+----------+

转让人删除受让人后，该表如下所示：

+---------------+----------+---------+----------+
| assignment_id | assignor | task_id | assignee | 
+---------------+--------------------+----------+
|       1       |    a1    |    t1   |    x1    | 
+---------------+----------+---------+----------+
|       2       |    a1    |    t1   |   null   | 
+---------------+----------+---------+----------+

现在，我需要一个仅当此特定task_id的每个记录在受让人列中都为空值时才返回true的查询。我到目前为止所写的内容：

SELECT DISTINCT task_id, nulls, total,

    (CASE
        WHEN nulls = total then true
        ELSE false
    END) unassigned
    
FROM

(
SELECT task_id, 
(SELECT count(*) FROM assignments b WHERE assignee IS NULL AND b.task_id = a.task_id) 'nulls',
(SELECT count(*) FROM assignments b WHERE b.task_id = a.task_id) 'total'

FROM assignments a 

WHERE assignee is NULL) c

结果是：

+--------------------------------------+
| task_id | nulls | total | unassigned | 
+--------------------------------------+
|    1    |   2   |   2   |     1      | 
+---------+-------+-------+------------+

关于改善查询或完全取代查询的任何建议吗？

Answer 1

如果我正确地遵循了您的规定，则可以使用聚合：

select assignment_id,
    count(*) - count(assignee) as nulls,
    count(*) as total,
    case when count(assignee) = 0 then 1 else 0 end as unassigned
from assignments
group by assignment_id

如果您要过滤未分配的分配，则可以在查询的末尾添加having子句：

having count(assignee) = 0

Answer 2

我认为您只需要两列：task_id和null中带有assignee的行数。
因此，group by taskid并在having子句中设置条件：

select task_id, count(*) total
from assignments
group by task_id
having max(assignee) is null

条件having max(assignee) is null仅返回taskid列中只有null个的assignee个。

仅当其所有记录在特定列中都为空时才查询返回true

2 个答案: