sql 行值作为列？

Question

我有一张这样的桌子：

| calendar_week | task       | person |
| 1             | cooking    | Bernd  |
| 1             | cleaning   | Julia  |
| 2             | carwashing | Bernd  |
| 2             | cleaning   | James  |

是否有可能像这样获得 output：

| calendar_week | cooking | cleaning | carwashing |
| 1             | Bernd   | Julia    |            |
| 2             |         | James    | Bernd      |

所以不同的任务应该成为列并且任务是“随机的”。

Answer 1

解决方案之一是使用连接。

select t1.tableA, coalesce(t2.person, '') as cooking
    , coalesce(t3.person, '') as cleaning
    , coalesce(t4.person, '') as carwashing
from (select distinct calendarweek from tableA) t1
left join tableA t2 on t2.calendarweek = t1.calendarweek and t2.task = 'cooking'
left join tableA t3 on t3.calendarweek = t1.calendarweek and t3.task = 'cleaning'
left join tableA t4 on t4.calendarweek = t1.calendarweek and t4.task = 'carwashing'

Answer 2

您可以使用条件聚合：

select
    calendar_week,
    max(case when task = 'cooking' then person end) cooking,
    max(case when task = 'cleaning' then person end) cleaning,
    max(case when task = 'carwashing' then person end) carwashing
from mytable
group by calendar_week

这适用于固定的任务列表，因为 SQL 查询必须返回固定的、预定义的列列表。 如果您要处理数量不可预测的任务，那么您将需要使用动态 SQL，或者，正如草莓所评论的那样，在您的应用程序而不是数据库中处理旋转。