Postgres 表选择多列并将结果（列）动态转换为行 - 将列转置为行

Question

我有一个如下类型的表（Damage）结构

  Rowid    damageTypeACount      damageTypeBCount damageTypeCCount
  1        23                     44              33

而且我还需要读取这些表行，结果为（标题、id 是手动设置的），有点将列转换为行，但具有附加属性

  id           damagecountsum          label
    1           23                  Damage Type A
    2           44                  Damage Type B
    3           33                  Damage Type C

我做了以下查询，它有效，但想知道是否有更好的方法

     SELECT 1 as id,
             damageTypeACount as damagecount,
            "Damage Type A Count" as label
     FROM Damage where rowId=1
      UNION ALL
     SELECT 2 as id,
            damageTypeBCount as damagecount,
            "Damage Type B Count" as label
     FROM Damage where rowId=1
     UNION ALL
     SELECT 3 as id,
           damageTypeCCount as damagecount,
            "Damage Type C Count" as label
     FROM Damage where rowId=1

上述查询按预期工作，但我想知道是否可以在单个选择语句中将列转换为行

Answer 1

您可以使用横向连接进行反透视：

select x.*
from damage d
cross join lateral (values
    (d.rowId, d.damageTypeACount, 'Damage Type A'),
    (d.rowId, d.damageTypeBCount, 'Damage Type B'),
    (d.rowId, d.damageTypeCCount, 'Damage Type C')
) as x(id, damagecount, label)

这会将原始id 重新影响到每个生成的行。你也可以用row_number()生成新的id：

select row_number() over(order by id, label) id, x.*
from damage d
cross join lateral (values
    (d.rowId, d.damageTypeACount, 'Damage Type A'),
    (d.rowId, d.damageTypeBCount, 'Damage Type B'),
    (d.rowId, d.damageTypeCCount, 'Damage Type C')
) as x(rowId, damagecount, label)

如果需要，您可以使用where 子句过滤结果集：

where d.rowId = 1