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sql - 每行到列值

问题描述

我正在尝试创建一个视图，该视图显示第一个表的列以及第二个表的前 3 条记录，按日期在 1 行中排序。

我尝试使用子表的偏移量选择特定行并加入主表，但是当加入查询结果时按日期排序，没有

WHERE tblMain_id = ..

加入 SQL 的子句返回错误的记录。

这是 sqlfiddle 示例：sqlfiddle 演示

主函数

| id | fname | lname | salary |
+----+-------+-------+--------+
|  1 |  John |   Doe |   1000 |
|  2 |   Bob |  Ross |   5000 |
|  3 |  Carl | Sagan |   2000 |
|  4 | Daryl | Dixon |   3000 |

tblSub

| id |           email |  emaildate | tblmain_id |
+----+-----------------+------------+------------+
|  1 |   John@Doe1.com | 2019-01-01 |          1 |
|  2 |   John@Doe2.com | 2019-01-02 |          1 |
|  3 |   John@Doe3.com | 2019-01-03 |          1 |
|  4 |   Bob@Ross1.com | 2019-02-01 |          2 |
|  5 |   Bob@Ross2.com | 2018-12-01 |          2 |
|  6 |  Carl@Sagan.com | 2019-10-01 |          3 |
|  7 | Daryl@Dixon.com | 2019-11-01 |          4 |

查看我正在努力实现的目标：

| id | fname | lname | salary |       email_1 | emaildate_1 |       email_2 | emaildate_2 |       email_3 | emaildate_3 |
+----+-------+-------+--------+---------------+-------------+---------------+-------------+---------------+-------------+
|  1 |  John |   Doe |   1000 | John@Doe1.com |  2019-01-01 | John@Doe2.com |  2019-01-02 | John@Doe3.com |  2019-01-03 |

我创建的视图

| id | fname | lname | salary | email_1 | emaildate_1 |       email_2 | emaildate_2 |       email_3 | emaildate_3 |
+----+-------+-------+--------+---------+-------------+---------------+-------------+---------------+-------------+
|  1 |  John |   Doe |   1000 |  (null) |      (null) | John@Doe1.com |  2019-01-01 | John@Doe2.com |  2019-01-02 |

标签： sqldatabasepostgresqltransposecrosstab

解决方案

您可以使用条件聚合：

select m.id, m.fname, m.lname, m.salary,
       max(s.email) filter (where seqnum = 1) as email_1,
       max(s.emailDate) filter (where seqnum = 1) as emailDate_1,
       max(s.email) filter (where seqnum = 2) as email_2,
       max(s.emailDate) filter (where seqnum = 3) as emailDate_2,
       max(s.email) filter (where seqnum = 3) as email_3,
       max(s.emailDate) filter (where seqnum = 3) as emailDate_3
from tblMain m left join
     (select s.*,
             row_number() over (partition by tblMain_id order by emailDate desc) as seqnum
      from tblsub s
     ) s
     on s.tblMain_id = m.id           
where m.id = 1
group by m.id, m.fname, m.lname, m.salary;

这是一个 SQL 小提琴。

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