Sql 如何将列名转换为多行？

我有一个遗留数据库，其中包含一个具有多个布尔类型列的表。例如：

表1

id name           has_lights has_engine has_brakes  has_tyres can_move
1  bullock_cart   false      false      false       true      true
2  car            true       true       true        true      true
3  tank           true       true       true        false     true

我想为Table1编写一个SQL查询，以获取id和名称以及属性（由列的名称表示），这些属性都是true

预期输出：

id name        attributes
-- ----        ----------
1 bullock_cart has_tyres
1 bullock_cart can_move
2 car          has_lights
2 car          has_engine
2 car          has_brakes
2 car          has_tyres
2 car          can_move
3 tank         has_lights
3 tank         has_engine
3 tank         has_brakes
3 tank         can_move

我写道：

SELECT id, name,
CASE
  WHEN has_lights THEN 'has_lights'
  WHEN has_engine THEN 'has_engine'
  WHEN has_brakes THEN 'has_brakes'
  WHEN has_tyres THEN 'has_tyres'
  WHEN can_move THEN 'can_move'
END
FROM TABLE1;

但这只为表1中的每一行获取第一个匹配属性（通过CASE-WHEN）

以我想要的格式检索数据的正确方法是什么？如有任何意见/帮助，将不胜感激

注:

• 表结构并不理想，但这是一个遗留系统，我们无法修改模式
• 嵌套查询只要不太慢就可以了——比如说对于上面的示例（我了解慢速查询中匹配行/列的数量）

---

最简单的方法是

union all

：

select id, name, 'has_lights' as attribute from t where has_lights union all
select id, name, 'has_engine' from t where has_engine union all
select id, name, 'has_brakes' from t where has_brakes union all
select id, name, 'has_tyres' from t where has_tyres union all
select id, name, 'can_move' from t where can_move;

如果您有一个非常大的表，那么横向联接可能更有效：

select t.id, t.name, v.attribute
from t, lateral
     (select attribute
      from (values (has_lights, 'has_lights'),
                   (has_engine, 'has_engine'),
                   (has_brakes, 'has_brakes'),
                   (has_tyres, 'has_tyres'),
                   (can_move, 'can_move')
           ) v(flag, attribute)
      where flag
     ) v;

---

您可以使用

UNION ALL

：

SELECT name,'has_lights' as attributes FROM YourTable where has_lights = 'TRUE'
UNION ALL
SELECT name,'has_engine' as attributes FROM YourTable where has_engine= 'TRUE'
UNION ALL
SELECT name,'has_brakes' as attributes FROM YourTable where has_brakes = 'TRUE'
UNION ALL
SELECT name,'has_tyres' as attributes FROM YourTable where has_tyres = 'TRUE'
UNION ALL
SELECT name,'can_move' as attributes FROM YourTable where can_move = 'TRUE'

---

这与精彩的查询非常相似：
由于

值

表达式可以独立运行，因此只需稍微缩短一点

SELECT t.id, t.name, v.attribute
FROM   table1 t
JOIN   LATERAL (
   VALUES (has_lights, 'has_lights')
        , (has_engine, 'has_engine')
        , (has_brakes, 'has_brakes')
        , (has_tyres , 'has_tyres')
        , (can_move  , 'can_move')
   ) v(flag, attribute) ON v.flag;