将键值列分组为一行

我正在尝试从在双列中存储键值对的 SQLite 表中提取数据。例如，用键foo, bar, man, and row，该表将如下所示：

| _id | external_id | key  | value |
|-----|-------------|------|-------|
| 1   | 12345       | foo  | cow   |
| 2   | 12345       | bar  | moo   |
| 3   | 12345       | man  | hole  |
| 4   | 12345       | row  | boat  |
| 5   | 67980       | foo  | abc   |
| 6   | 67890       | bar  | def   |
| 7   | 67890       | man  | ghi   |
| 8   | 67890       | row  | jkl   |
  

我想执行一个查询，给出每个external_id在一行中，键作为列，值作为行。像这样：

| external_id | foo |  bar | man  | row  |
|-------------|-----|------|------|------|
| 12345       | cow | moo  | hole | boat |
| 67890       | abc | def  | ghi  | jkl  |
  

我能想到的唯一解决方案是每个键的连接：

SELECT a.external_id, b.foo, c.bar, d.main, e.row 
FROM myTable AS a
LEFT JOIN 
  (SELECT external_id, key AS foo
   FROM myTable
   WHERE key="foo") AS b
   ON a.external_id = b.external_id
...
LEFT JOIN 
  (SELECT external_id, key AS row
   FROM myTable
   WHERE key="row") AS e
   ON a.external_id = e.external_id
GROUP BY a.external_id

有一个更好的方法吗？

另一个可用的选项是使用条件聚合:

SELECT external_id,
       MAX(CASE WHEN key = 'foo' THEN value END) AS foo,
       MAX(CASE WHEN key = 'bar' THEN value END) AS bar,
       MAX(CASE WHEN key = 'man' THEN value END) AS man,
       ... etc
FROM mytable 
GROUP BY external_id